We have to readjust our mindsets a bit here: people didn't necessarily need to launder all of their clothes the way we do today. Instead, it was normal (for more than a thousand years - my knowledge isn't so encyclopedic before about 1500 or so, but I know of this tradition going back at least to the Anglo-Saxons) to wear a fairly plain garment under the clothing that could be seen, in order to protect it from the sweat and oil of the body. Men's shirts and women's shifts of the 18th century were made of linen, usually cut in fairly basic geometric shapes that made use of the woven selvage of the fabric to form unfrayable straight lines, and sewn together with firm stitches that would hold up to repeated rough laundering. Shirts were made with a standing collar that protected the neckcloth, and sleeves that went to the wrist underneath a coat; women's shifts instead had a lower neckline cut out of the body, to match the neckline created by the gown and stomacher worn throughout most of the century, and shift sleeves didn't go much past the elbow, as long sleeves were not typically worn by women.

These pieces were made out of linen at all different levels of quality, from very fine and almost sheer cloth to unbleached tow that would have to be broken in somewhat through both laundering and repeated wear to be comfortable. These comprise some of the earliest types of ready-made clothes, since they didn't have to fit the body in a more than cursory way, though they were also often made at home by women who may not have had the skills to cut out the pieces for more close-fitting outerwear made from more expensive fabric, and could be bought at second-hand clothes dealers (who were plentiful in cities and cheap).

In the early 19th century, cotton began to edge out linen as, very basically, cotton production was overtaking it. With slave labor and the cotton gin, the fluff itself was cheap and easy to process, while linen required "retting", i.e. being allowed to rot in water for a certain number of weeks, followed by a combing with metal "heckles". The American textile mills of this period were largely set up to deal with cotton, and those of England were trending that way themselves. Cotton then became cheaper than linen (though it had been affordable for a very long time already), and made more sense for use with these undergarments. There isn't a strict correlation, but as cotton became more commonly used this way, the cuts of the undergarments themselves changed to be less chary of waste. The "Corazza shirt" for men appeared in the 1840s; it was very different from the previous type, as it buttoned down the back and had an arrangement of pleats in the front, which likely is related to the growing trade in ready-made clothing that was more complex/difficult to make than those that were done at home. (Elizabeth Gaskell wrote of a woman dealing with "the puzzle of devising how [Corazza shirts] were cut out" in the 1851 Mr Harrison's Confessions.) Shifts - now called chemises - took on a number of different forms that often involved gathering to a yoke and the insertion of cheap machine-made laces or machine-embroidered ribbons. However, though they were fancier, they were still intended to be unseen and to be replaced and washed as needed.

Okay, so, what does "as needed" mean? That's tricky, because we can't tell from the extant pieces of clothing, and it wasn't regularly discussed. In the early modern period, there's evidence that people began bathing less specifically because they could have more changes of linen, which took care of the issue. (See "The Body, Appearance, and Sexuality", by Sara Grieco in A History of Women in the West: Renaissance and Enlightenment Paradoxes, Harvard University Press, 1992, if you don't believe me.) Throughout the 18th and 19th centuries, the ideal was to change at least every day, perhaps more often if one had gotten sweaty doing something physical or if the day were hot - having clean clothes made you more comfortable, obviously, and having crisp, fresh linen around the neckline of your clothes or peeking out from your sleeves was a symbol of both your social status and your moral standing - fiction and non-fiction of the 18th century takes note of the state of individuals' linen, drawing conclusions from the contradiction between someone's greasy, dirty shirt and other fine things they might have. Dirty linen was also thought to cause or attract illness by trapping impurities and "poisons" next to the skin.

The majority of people had a decent number of changes of body linen in their wardrobes, because these items were a necessity. In late 18th century France, for instance, three-quarters of the people in the bourgeoisie and artisan classes owned between ten and thirty shirts or shifts. Garsault's 1771 Art of the Linen-maker described a noble trousseau containing 72 shifts, which would allow for multiple changes per day as well as several to wear while others were being laundered! The very poor, however, might own only two shirts/shifts, wearing one while the other aired out or sat - or just one, if they were truly destitute. Paupers were typically given an outfit containing only one shirt or chemise upon entering a charitable institution, and they would rewear them until given fresh ones; as the laundries in these places were often inadequate, the "clean" one might not be much better than the one they were already wearing.

With outer clothing, there was not much objection to wearing the same thing repeatedly as there wasn't an implication that the proper laundry wasn't being done: the only issue might be that you appeared to have few items of clothing, which obviously would show your class/wealth. However, it's likewise difficult to say exactly how frequently individual coats, breeches/trousers, or gowns were worn, since inventories made after an individual's death rarely make it clear which clothing has been held onto for sentimental or other reasons, and which were actively being used. The 1747 inventory of the clothes and accessories of Mary Churchill, 2nd Duchess of Montagu, for instance, tells us a lot and a little. She had 25 shifts (probably all in regular use, since there would be little reason to hold onto worn-out ones) and 27 gowns of various fabrics, from seersucker to silk - were half for winter and half for summer? Were many held onto from her early life, or were they in regular rotation? We don't know. Barbara Johnson, an 18th century Englishwoman, kept an album with snippets of all of the fabric she purchased and had made up into clothing, usually with a note of the year and what was made, which is both useful and unuseful in a similar way: we can tell her habits of consumption, but not how often she got rid of or remodeled anything. When she bought two new mourning outfits in 1759, was she wearing those two day in and day out, or did she still have and use her mourning outfit from 1756, or the one from 1753, or even the ones from 1751?

We do know that many people, particularly in the 19th century, might change their outer clothes more than once a day. To quote from a recent answer of mine:

The average man didn't really do any changes, while an affluent businessman might change out of his three-piece suit for dinner. For women, housedresses (loose, unlined, washable dresses) were very common during this time, and even an average woman would probably wear one while doing the housework to spare her normal clothing, which she could change into in the afternoon; moving up the social scale, an affluent woman might wear a housedress early in the morning, change into a different dress for the bulk of the day, and put on a dinner dress at night. Anything else would be put on on an as-needed basis, the same way that people do today - sports clothes for sports, riding clothes for riding, swimsuits, etc.

What a terrific rundown from u/chocolatepot. The only thing I have to add is about the fibers themselves.

Until quite recently, wool (in Europe and colonies) was THE fiber for cold weather clothing. Wool isn’t “self cleaning” per se but is much more dirt resistant than many fibers we wear today, including modern wools both very fine/thin and lower quality/rough. One interesting quality of wool is that it can be aired out, not something that would work with those polyester leggings you’ve got there. So the everyman suit, ladies “walking” skirt suits, outerwear, and even athletic clothing of the day was all made of wool, and could be aired at the end of the day. The clothing would be dry and odor free by morning. Some spot cleaning and a change of linen or cotton underthings and you’re fresh as a daisy. Think of it like your leather jacket. When’s the last time you washed that? Also while wool can be laundered at home, sending clothing out to be specially laundered was an option at least in larger cities. It’s not so much the washing that is annoying than the pressing. Improperly dried and pressed wool can become felted (ruined) or misshapen.

Yes, finery was made of silk, which stains quite readily and is annoying to launder. All I know about this kind of frippery is that when garments with fine trims, such as beaded panels, fur collars, lace cuffs etc, had to be laundered, the part was removed and laundered separately. It was then reattached with very small hand stitches - this could be done quickly.

On to linen. Side note, the French term lingerie does derive from linen, so we might think lingerie as sexy bras and panties, in the last it just meant the boring white underclothing everybody wore. Linen is a great choice for wearing close to the body for a few reasons. It is resistant to staining (this is why dye fades so fast from linen) and can be easily sunbleached back to white. So even when old, it can look white like a newer garment. Second, linen is also great with sweat management. Third, linen being white could easily be starched. Even sheets used to be starched. As the starch became soiled, it could be brushed away. This squeezes another wear or two out of a garment or so I’m told. I don’t know if underclothes were starched or just shirts/chemises and bedclothes.

Finally onto cotton. There’s nothing too special about the fiber itself but just an interesting note that there was a distinction around the turn of the century between “wash dresses” which were invariably cotton, and other kids of dresses. These days, nearly everything is a wash-and-wear garment that we launder at home, and that we wear at home, at casual events, and at work. We don’t even iron (admit it). A wash dress would be for a woman to wear while working in and around the house. It is probably printed, to hide stains but look cheery. A woman would not wear a wash dress out in public unless she was particularly harried. A fancy apron or a coat could be worn over a wash dress. (Last word note. Aprons used to be called naprons. A napron sounds like an apron.... which is what the word became. Weird right?)

All the seams and edges of wash dresses were made for durability. Any trim like belts, or fasteners, or shaping details, would ideally be flat, removable, or easy to press. Everything used to dry on the line, and wrinkles are the sadly inevitable result. The quick and attentive housekeeper/housewife would bring in clothes when they were just damp, and stretch and press them right away. Leave something out to dry to a crisp and you’ve got more of a pressing struggle. The last thing you want to do at that point is press pleats or fussy ruffles, so wash dresses are plain. I believe linen undergarments were also treated the same way in design and in laundering stage.

Up until the 1930s, it was still common to have only a few garments (working class people anyway) and just switch back and forth. Ladies who found themselves city jobs, as telephone operators for example, could get by with one or two dark dresses. They could still change their look every day by swapping the blouse, which would be seen as collar/cuffs, or by downsizing the blouse to only a collar or cuff. A button or hook/eye would hold it together, but it would not need to be attached to the dress. Laundering blouses and cuffs is pretty easy in a tiny rental room. Very practical. Small handcrafts like bobbin lacemaking, crochet, tatting, embroidery etc were a great way to come up with inexpensive new accessories as well as an entertaining hobby.

And finally, I have references up to the 1920s-ish that were still recommending ladies air out their clothes every day to be worn the next day. While changing the underwear every day was at this point almost a given, it was suggested underclothes be at least aired every day and not worn longer than 2 days. Ladies underwear at the time was loose shorts of various types or kinda teddy things worn with bralettes or corsets. Ladies undies were not tight like they are today, if that changes anyone’s mind about how gross a two-day pair might be.

Hth!