When discussing a strict, by-the-book approach, the first stop of course would be the manual for the weapon in question. To use the American G.I. as an example, 'FM 23-5 Basic Field Manual: U.S. Rifle, Caliber .30, M1' lays out in detail the procedures for cleaning the weapon in various situations. In terms of 'proper' procedures, where full cleaning could be done, it lays out several situations in Section III. The procedure for Garrison and Camp (10) use for example dictated the use of Sperm oil for comparable for its superior protection, while when cleaning preparation for use on the Range you would substitute lubricating grease (11). After use on the range (12), not only was a long post-shooting cleaning dictated, but if you weren't going to the range daily, "[the bore] should be cleaned in the same manner for the next 3 days." In the field, of course, there often isn't time for such things, so only the key points are emphasized for care and cleaning at the range or in the field, which I'll repeat in full (13):
The rifle must be kept clean and free from dirt and properly lubricated with lubricating grease. To obtain its maximum efficiency the following points must be observed:
a. Never fire a rifle with any dust, dirt, mud, or snow in the bore.
b. Keep the chamber clean and free from oil and dirt.
c. Never leave a patch, plug, or other obstruction in the chamber or bore. Neglect of this precaution may result in serious injury.
d. If the rifle gives indications of lack of lubrication and excessive friction, apply additional lubricating grease to the parts listed in paragraph 11 a (5). Excessive friction exists if the empty cases are being ejected to the right rear, and grease should be applied at the first opportunity as failures to feed and eject will occur if the condition is not corrected.
e. Keep a light coating of oil on all other metal parts.
f. Remove the carbon from the gas cylinder plug and the piston head when necessary.
g. In general it should not be necessary to remove any of the parts of the rifle in the field for cleaning except the trigger housing group and the gas cylinder plug. However, if the mechanism becomes very dirty the rifle may be disassembled into its three main groups and the necessary cleaning and lubricating accomplished.
h. During range firing, a selected and qualified man should be placed in charge of the cleaning of rifles at the cleaning racks or tables.
13(g) is perhaps the most to the point part here, emphasizing that in the field, minimal disassembly was expected except when really needed. Further points for 'immediate care' are also covered in Section VI regarding how to deal with stoppages, but as the summary table makes clear enough, many solutions were not possible in the field under fire.
But the above, of course, is only a manual. It doesn't say all that much about how soldiers actually dealt with cleaning in the field. In the best case scenario, of course, they didn't have to! If being removed from the line for a period of time, the weapons would be turned into the armorers, who would provide the most thorough of cleanings, not to mention replacing any parts which might be showing undue wear and tear. But that was a luxury that was not often possible, and even armorers behind the lines could be pressed for time by circumstance. Members of the 101st, for instance, were afforded such an opportunity only to have R+R cut short and weapons quickly reissued (he doesn't state whether all had been processed) when the Germans had the temerity to attempt their push into the Ardennes.
More generally, soldiers would simply take any assured downtime to spend some time doing a basic cleaning of their weapon, which for the most part did the trick. Many weapons, like the M1 Garand, even had a basic cleaning kit that was kept in the butt of the gun. During lulls in the campaign, cleaning weapons was, if anything, a way to break the daily monotony, although maintenance of course was a generally expected duty regardless - "Maintenance of weapons was like eating—it was essential for survival" as one Marine put it. When coming off the line, even if not getting the benefit of an armorer, a more thorough cleaning for inspection was also to be expected. Likewise, with the expectation of going into action soon, fastidious cleanliness was expected, to of course minimize the chance of something going FUBAR in action. Although that didn't stop things from happening anyways, which had to be dealt with best they could. On the beach at Iwo Jima, Chuck Tatum, a young Marine, recalled having to deal with a jammed machine-gun:
I slapped my machine-gun tripod on the deck and Steve snapped our weapon into place. I lifted open the breach and Steve handed me the ammo belt. I slammed the breach over the belt and pulled the bolt back, chambering the first round with an authoritative click. I tried to fire the first burst. Nothing happened!
The damn gun wouldn’t fire because the breach was full of sand and grit. We had carefully wrapped our weapon in a protective green Marine bath towel intended to prevent crap from jamming the bolt.
It hadn’t worked.
“Why in the name of hell did this have to happen now?”
I threw the breach open again and rolled over on my side while Steve opened my pack and got the cleaning gear out. I used a toothbrush to carefully clear away the sand fouling the breach. This took less than thirty seconds, but seemed like a lifetime.
Of course, the environment, and the weapon itself, played into that a good deal. There are far fewer complaints about keeping weapons functional in memoirs from the ETO than those island-hopping the Pacific where it is an enduring theme, and the humid weather was hardly a friend to the metal surfaces of the guns. Similarly, the sands of the North African desert got into everything, and GIs found that their M1s required a minimal amount of lubricant to avoid problems, and still needed twice-a-day cleaning. Further, some firearms in particular gained a reputation. The Reising, a submachine-gun manufactured to in addition to the better known "Tommy Gun", and grabbed up by the USMC trying to bolster their meager stocks of SMGs early in the conflict, earned the nickname of the "Rusting" from the Marines unfortunate to be issued with them. Recalled one Marine of the unwanted hunk of metal:
Frequent cleaning didn’t help. You could swab and oil the Reising vigorously, twice a day if you had the time and inclination, and the next morning you would find brown rust eating away at the barrel. [...] I would venture to say that nearly every Reising carried by Marines who served on Guadalcanal was “lost” at some point. Since the Reisings on Guadalcanal constituted almost the entire inventory of that weapon in the military’s arsenal, their disappearance pretty much ended its service career.
Perhaps slight hyperbole, but the Marine's attitude certainly seemed to be that the only way to keep a Reising clean was to just get rid of it. On the other end of the spectrum, Marines fortunate enough to be carrying the M1 Carbine were pleased as punch on that front, as its' .30 ammunition, unlike most of that period, used a non-corrosive primer. Not that regular cleaning wasn't necessary still, but it could take more use before fouling became an issue (although on the flipside, the ammunition itself could corrode, and if left in the magazine for long periods of time, that could cause jamming.
As for the M1, the bread-and-butter of the American Grunt, it had its teething problems (see above in North Africa), but the soldiers and Marines, for the most part, were happy with its relative easy of care, finding it to be quite reliable as long as you were regular with cleaning. Even in rough conditions, keeping it clean ini the downtime generally ensured it would shoot fine in action. In snow or freezing rain, care needed to be taken to avoid build-up in the operating-handle slot, which could jam the action if frozen up, but this was easily solved by wrapping something around it. Further, in wet conditions - especially amphibious assaults - a popular measure, seen in movies like Saving Private Ryan, would be to put a condom over the barrel until ready to fire.
In short, keeping your weapon clean was an imperative, and while there is no guaranteed timeline soldiers would clean their guns on, it can safely be said that "often" is the answer. A functioning firearm was a necessity, to say the least, and a clean gun was (usually) a functioning one. While cleaning your weapon under fire would only be done if absolutely necessary,^1 if there was available downtime, giving at least a basic one-over was a near daily ritual.
Sources Consulted:
1: I put this only as an aside, but a humorous one, as it is neither exactly history or WWII, but it is conventional wisdom in CW re-enactor circles that when a barrel was too fouled up during combat to load again, soldiers would piss down it, as the ammonia in the urine would clean it out. As such, there are many friend-of-a-friend stories about a re-enactor who pisses down their barrel to trying to be a bit too authentic... and then sprays a mist of pee on everyone the next time they fire.
A truly comprehensive answer to this question would require going through each branch's field training and various weapon systems, for each of the countries with infantry actively engaged in the field, in both major wars. Furthermore, while some small arms were issued in both world wars (i.e. the G98 - K98 carbine version), many of the small arms and corresponding ballistics evolved between 1918 and 1939. Indeed, Winchester's "non-corrosive" primers for the American .30 cartridges were well-represented among the infantry in WWII (I put non-corrosive in quotes because, while the powder/propellant was modern and considered non-corrosive at the time, the cartridge would not be considered non-corrosive today). As such, for quite a few weapon systems, the answer would vary depending on the war in question (and, actually, the specific year of that war).
On top of that, location of use is another metric with fairly significant influence on the answer to your question (i.e. whether we're discussing conditions during muddy November at Verdun, August in Sicily, the rainy season in Palau, winter in Belgium, or a windy dune range in Algeria). The wars were fought throughout, more or less, almost every climate biome on earth. Thus, depending on the environment, the affect of the elements created varying manifestations upon ammunition, internal ballistics, wood stocks, internal carbon residue, metal-on-metal actions, the type of solvent used to swab a barrel depending on temperature, etc.
With respect to your secondary question, we'd have to establish an operative definition of "in combat." For instance, it's fair to say there were German and/or French units on the front and technically "in combat" (being actively engaged with, at immediate risk from, and in very close proximity to enemy combatants) during the months of March-April at Verdun. Likewise, there were Marines in the Pacific theater who'd spend several days in a row in positions being constantly assaulted and/or actively carrying out assault operations themselves.
That could end up being a 50 page paper. So, I can answer the question with respect to one of the small arm platforms used all over the world during WWII - the M1 Garand.
This rifle shot a .30-06 cartridge, known more readily in the 1930s as just the "M1" or the ".30 cal" (before the proliferation of the M1 carbine and other variations of the .30 cal cartridge). This is well-known to be the standard issue infantry rifle for American forces during WWII. Most of these .30-06 rounds were 152 grain loads, and used a nitrocellulose propellant. The specific propellant used for the M1 Garand was (or, "is," as its actually still produced for home reloading) known as IMR 4895, which included dinitrotoluene, graphite, diphenylamine, potassium sulfate, etc. This ammunition was very corrosive, but corrosive ammunition is well-known for long-term storage and is fairly reliable.
To hone in on the "how" of your question, it really depended on the circumstances at hand. To the infantry soldier of WWII using an M1 Garand, they didn't know what "corrosive ammunition" was, to them, it was just regular ammo. This is an important point in regard to your overall question - at the time, ammo was ammo. The corrosive, carbon-heavy residuals of the bullet explosions inside the guns were, to soldiers / marines carrying the M1 Garand, likely superior to most of the other small arms they'd have been exposed to up to that point, and certainly better than other weapons in the field at that time. But the full field-strip cleaning vs. quick fix cleaning was relatively the same to how we'd do it today. Because of the properties of the compound in IMR 4895, they likely had to clean the barrels and actions with more frequency than one would have to today using modernized synthetic lubricants and preventative oil application pre-use. That said, most of the reviews of the rifle from servicemen were positive with regard to cleaning requirements.
Under non-combat situations: At that time, the War Department was responsible for producing the training manuals for field stripping / full service of the M1 Garand. The way this rifle was fully-cleaned (with time for disassembly) just how you'd clean it today: You pop up the trigger guard, and remove the entire housing assembly directly from the stock of the rifle. This unlocked the barrel and receiver group from the stock, as well, which you lift right off the stock. You can then remove the follower arm, spring, operating rod, and bolt. The cleaning kits at that time were fairly basic, and the primary components were the collapsible cleaning rod, several changeable tips (patch loop, bore brush), grease and oil. One of the great aspects of the M1 Garand was the utilization of grease instead of sole dependence on oils, like the auto / self-loading weapons before them. Grease / synthetic lubricant is much lest quantity-demanding, and doesn't require an in-system reservoir. One author I read claimed the amount of oil needed to service a Thompson .45 could - in volume - could service 12 M1 Garands. So, in sum, when an infantry soldier/marine had time, field stripping the weapon and servicing it was incredibly similar to how infantry soldier/marine serviced the basic Springfield M14 all the way through the 1990s (it's actually still in service today, but the stocks - for the most part - have evolved quite a bit).
In more stressful, time-sensitive combat scenarios where you have a rifle malfunction that prohibits full use of the weapon, the cleaning options were more basic: remove the clip (M1 Garands used clips, not magazines), lock the action back, blast your bore brush through the barrel, follow up with a cleaning patch, reload clip, chamber round, fire.
With respect to the question about "how often" they cleaned their weapons in combat, the answer for the M1 Garand was, famously, not that often. Two of the reasons this rifle was so popular was because of (a) the reliable action (which was one of the first self-loading rifles with unsealed open friction surfaces to receive such global praise), and (b) the use of grease, which increased longevity and kept a lubricant in one place, without constant re-application like automatic small arms at the time often required via a reservoir. So, again, for the Garand, the frequency with which they had to be cleaned "in combat" was lower than most at that time.
Again, another factor to take into consideration here is location of combat. For U.S. Airborne soldiers fighting in the winter in Belgium during WWII, there are diary entries and service reports that the Garand's grease could freeze over night or during prolonged periods of extreme cold and non-use. Thus, some soldiers would actually strip them if anticipating some down time and wipe down as much grease as they could to avoid said freezing. Also, there are reports of soldiers peeing on their Garands to unfreeze them if necessary (Marine Theo McLemore wrote about having to pee on his BAR to unfreeze it in Korea). At the same time, with exceptionally hot weather in WWII, the grease on an M1 Garand could "run" when it was overheated, and it would require the opposite - increased application of grease onto the moving parts / unsealed friction bearing surfaces of the action / bolt.
With respect to other weapons used by the world's infantry in WWI and WWII, again, it all depends. Bolt action rifles (the standard issue for most infantry who fought in WWI) are much simpler mechanisms than self-loading rifles, and require less on-the-spot maintenance than autoloading small arms. WWI was a unique time, as well, as many of the small arms in the field were undergoing their first, legitimate, sustained combat / field testing since implementation. Thus, non-bolt-action weapon systems evolved and/or were replaced, even during the duration of the war, at a much faster rate.
I wish I had the time and space to answer your question with regard to all small arms used in both wars, in all environments and all situations. I hope this helped explain some of it.
Sources:
Military small arms of the twentieth century, 7th Ed. Ian Hogg & John Weeks
Official Training Film T.F. 21, 1018, War Department, 1943
Edit: I apologize for the spelling errors
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