I have several concrete mixing pans and a kiddie pool and a half rooftop carrier. The smaller the pan and the more ducks get changed more often or the hose put in and about half the water replaced.....every 2-4th day. It depends on the heat, sun exposure etc. Usually I go by smell....if it starts to smell I change it right away, and I don't let it get green.In winter and cool times I don't have to change it as often.... Some of my ducks are messier than others. Some just get in , get wet and get out.Some spend lots of time in the pool...floating. Some drink while eating from pool dropping food in it, causing it to stink sooner. I had a 140 gallon pool which is out right now,that I only changed 2x a month in . summer , less in winter., I'd say I average 1-2 times a week, depends on weather....etc.