It depends danfrog, My one baby poops 2-3 times a week and my other one poops once a week. I drop them in the waterdish on wednesday and saturday, if no poop is seen an entire week, I give them a little bath.
My babies seem to catch on fast though, they are both pooping by themselves. So now I have 3 babies that are potty trained and my adult refuses too unless I make her, funny no? :P