I have the Drinkwell with the 6 Cup reservoir, and three cats. I refill it every 2-3 days, and tear it apart for cleaning about every 2 weeks.

When I do the 2-3 day refilling, I just refill the reservoir part, the stuff that is already in the bowl stays, but I think they go through water fast enough that is it refreshed and not old. I use my filtered refrigerator water to refill. Once I let it get too low, and it started making funny noises - probably not good. It is hard to carry to the sink with the bowl to full though, so I try and let it get down there before cleaning.