Personally O bathe daily!
The horses will, if they are sweaty get washed down, without shampoo, with warm water, after a ride.
During the winter when they are clipped out and fox hunting they will get bathed after a days hunting.
This is done with shampoo and hot water, it gets them clean quickly, the hot water is both more comfortable for them and opens the pores to allow any trapped mud to be freed.
I scrape them off and then rug them up for the night. The moisture wicks through the rugs and they are dry within 20 minutes.