As needed. I have a brown horse living in a humid climate. He has anhidrosis and his sweat, when he produces it, tends to be very lathery.
My guy has occasional allergic reactions to something in the soil in his field, and when he’s got hives, he feels better with a scrub with some oatmeal shampoo.
I also do a bubble bath prior to a body clip to start with a clean coat, and afterwards to get clipper oil off of him.
I wash between his hind legs on a more regular basis to clean lathered sweat, scrub legs regularly, and wash and condition the tail on a regular basis.
Now, since he’s a lousy sweater and he doesn’t cool himself well, I do a fair number of liniment rinses and alcohol baths for that nice tingly cooling feeling while that evaporates off the body.
To clean the body, well, I’m a pretty good groom.