A lot depends on what type of watering system you use. Those of us who use nipples refill when the level gets low. I normally change out my water every 2 weeks or so just to freshen it up, but have never seen any algae.
I have a little wading pool for them that they also tend to drink out of . I try to clean it out every 2-3 days. I'll be happy when it gets cooler and I can remove it so that the nipple waterers are their only choice.
Thanks for your reply's.
I have a galvanized, 2 gal waterer, actually I have two of them, and have been changing them out daily. That will end immediately. I have built the cookie tin heater for the winter.
My last cat would have fresh water and drink old water out of the plant trays.
This is what I use, but this picture was taken to show it overflowing, and it has been replace by one that works.