Try to avoid baths to at the most, once a month- but that might even be too much.. like said before, every 2 or 3 months is probably at most.
Cavy Tamer, LAP time is great bonding time. How do your pigs enjoy the baths? Do they like, play? you COULD just let them play in water if they enjoy water..
And amfeider, don't give your piggies baths a lot. If she gets dirty, just wash her with a wet washcloth to get her clean. My pigs are white, but they don't get dirty. I don't understand what she could get into to make her dirty?
On second thought, I just saw that you said "on her but and belly". Do you actually WASH her entire body? Like I said before, you could just use a wet washcloth if she gets her belly a little dirty or something. I don't think it matters if it's just a little bit of the body, it could still dry. But if you give her butt baths, that's what some people do often..