my mare is a palomino white mane and tail, I bathe her once a month or longer, depending on the weather. We typically hose our horses off after rides on nice days when misquitos aren't too bad and that keeps them pretty clean regularily. Plus I have a fly sheet on her.
If you're looking for a good shampoo I use White 'n' Brite and it is really good for cleaning up the lighter areas! You can mix it with water or use the concentrate directly on the horse for stains or mane and tail cleaning. It ran for about$30.00 but has lasted a whole year so far with over half the bottle left.