We have racehorses so it usually evolves at the very least, sponging them off every day(jogging) or bathing (training). We don't use soaps just warm water. The only time we use soap is at the racetrack after the race and we give them two baths. One with soap and one with rubbing alchohal and epsom salt combined in a large bucket of near hot water (helps with muscles and such).
Usually just to cool them off after a ride or on a hot summer day. Mostly I use a body rinse and sponge. I do shampoo their tails as needed.
I do, however, hose and clean their legs and feet every time they come in from the pasture or arena before they go into their condos. I never leave mud on their fetlocks and hoofs as it can contain bacteria. This way we avoid scratches and other nasty stuff. I get off the excess water with a towel and quickly brush a hoof conditioner which contains a disinfectant in it too.
I have a light grey mare, so she gets a 'real' bath (with shampoo) once or twice a month. I usually end up hosing her off about once or twice a week, and I brush her 5 times a week (every day that I see her).
I'd like to be able to get away with just brushing her, but she's really hard to keep clean so I usually end up having to hose at least part of her off.
It depends on the horse and the time of year. During the warmer months, my friend who owns a bay horse baths about every 2 weeks. I own a grey however and he gets a bath at least once a week during the summer. They get a bath before every show and most of them at our barn are bathed before they are clipped, its easier to clip a bathed horse.
I hose my horse off in the summer and late spring/early fall after every ride (summer mostly) and if he's really hot. Most of the time if they aren't really sweaty a sponge or a good brushing will do.
I bathe mine about twice a year. When I was showing though I'd bathe them before shows but now I don't so it's once or twice a year. I think this past year though due to me being home very little they were only bathed once. I do brush them often though.
I forgot to add...if they get really sweaty from a ride I do hose them off!