Well, I have a 5 gallon bucket with three drinking nipples and many more chickens, which DO have access to other sources of water since they range freely... However, when they were confined to the run, I had to clean the algae out of the bucket more than I had to refill it. I topped it off every few days in the hot weather just to ensure a constant supply, but it never got below one-third full in any case. A 5 gallon bucket lasts quite a while. Every two weeks I'd dump and clean it to get rid of the algae.
Hope that helps!
Try to put yours in a shaded place and if you have something other than a WHITE bucket, that would be good too. It will help keep the algae from growing. (So will the addition of a small amount of organic Apple Cider Vinegar.) Mine is placed in the sun, unfortunately, but I put a small yard umbrella up to help shade it.