How often do you clean your fountain?

A question for those of you with pet fountains. How often do you clean them? Also what kind do you have and how many animals share it?

I clean mine about every 3 or 4 days when it needs to be refilled. It seems to be getting dirtier faster since I got Polly. I think she's playing in it. I have 2 cats that share a Petmate Deluxe Fresh Flow.

I have a Cat-It, one cat, and I clean it once a week. This is also when it needs to be refilled.

cleaned once a day - refilled once a day!!! I have the Pet Fountain.

This is the only water bowl Loki dosen't play with

I've got a Petmate Fresh Flow, which I clean weekly with hot water, or vinegar and water. I generally have to refill it once or twice a week. Jamie has access to it, while ZsaZsa doesn't (it's in "his" part of the house). She's a bit weird when it comes to drinking: She doesn't like the pet fountains or water bowls, but will drink plenty from a plastic bucket that is half-filled. She balances on the rim, which makes me afraid that she'll fall in and drown (she only weighs 6 lbs., so the bucket is twice her size).