I realize this probably changed by country, and am not necessarily focused on one single country's system. Were there incentives for volunteering instead of being drafted? Also, if someone was captured as a POW but was liberated before the end of the war, did they rotate back into service or were they sent home upon their release?
I just can't imagine how demoralizing it must have been to not know when you might return from war and wonder if soldiers had a date in mind they could dream about returning home. Apologies if this topic has been covered before, I searched and quickly checked popular questions but didn't see one that addressed this.
It will vary by country, by year, and by service. Let's talk about the Pacific Theater, comparing apples to apples: the piloting corps of the U.S. Navy versus the Imperial Japanese Navy.
Generally, Japanese pilots flew until they were shot down or killed; American naval pilots were taken out of the front lines, rotated back to the U.S., and used to train a whole new generation of flyers.
The Japanese
Philosophically, pre-war Imperial Japanese Navy (IJN) planners favored quality over quantity, both in materiel and in manpower. This was the product of necessity and an unusual psychological blind spot.
It was the product of necessity, because the U.S. was richer, had greater industrial capacity, and had a larger population. IJN high command knew that a long war against the Americans was unwinnable. As such, the prewar Japanese war machine was designed around having better things-- in 1940, the Japanese had the biggest battleships, the best carrier aircraft, and a core of 2000 superbly-trained aircrew that made the Imperial Japanese Navy the most dangerous fighting force in the Pacific.
As for the psychological blind spot, Japanese planners believed that any war with the West would proceed similarly to the Russo-Japanese War (1904-5). There, the Russian Navy had sent their fleet straight to the main battle theater, were worn down by Japanese harrassment, and ultimately destroyed in a decisive battle far from home shortly after the initiation of hostilities. The Japanese expected the Americans to do the same as the Russians. (SPOILER ALERT: the Americans didn't.) As such, the Japanese never developed the infrastructure to replace the aircrew, technical staff, and damage-control personnel necessary to fight a long war.
In practice, this meant that the pre-war piloting corps was irreplaceable. In the interwar period, each individual Japanese carrier pilot took about two years to train, and once hostilities began there was no way to keep up with combat losses. The skilled core of IJN aces would rack up enormous kill counts, and when they died, their enormous knowledge of aerial combat died with them.
At the Coral Sea and Midway in 1942, Japanese flyers were still outmatching their American opponents, plane for plane. A good illustration of this is at Midway, where the Japanese defensive fighters shredded the American bomber corps time and time again, wiping out three squadrons of carrier-based torpedo bombers and one squadron of land-based bombers with minimal losses. (The American dive-bomber attack that finally sunk three of the four Japanese carriers only succeeded because the defensive fighter screen had been pulled out of position by the disastrous torpedo strikes.) Midway was not a crippling blow to Japanese naval aviation-- but it did mark the beginning of the end for the Japanese naval air service.
After Midway, IJN high command undertook a crash training program to put warm bodies in cockpits. But the new pilots were nowhere near the quality of the pilots that had been trained before the war, and so the Japanese carrier air force gradually declined as the Americans slowly cut the Japanese down by attrition. By the summer of 1944-- the appointed time for the long-planned decisive battle against the Americans in the Philippine Sea-- Japan could put men in cockpits, but their pilots were totally, utterly outmatched against the Americans. The battle of the Philippine Sea was known as the "Great Marianas Turkey Shoot" by American flyers. And with the death of most of their remaining trained aircrew, the Japanese were forced to resort to kamikaze tactics to hold off the American juggernaut.
-- continued in a second post --
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u/no-tea's contrast of naval aircraft pilots is awesome, but the question on my (and I suspect, many others') mind is that of the soldier, as in actual riflemen enlisted in the various armies. There's a passage in All Quiet on the Western Front where someone who has been deemed unfit for duty is awaiting some paperwork to be shipped back home (and sadly, catches a piece of shrapnel while waiting), but what about the rest of the unit? Were squads/platoons/companies relieved and at least allowed to rest behind the lines or were they simply parked in the trenches until every last man succumbed to PTSD shell shock?
I think OP is referring in terms of the length of deployments in the modern US service, where an enlistedman is posted to an active zone for a certain number of months/years, and then sent back home despite the continuation of hostilities.