I have a 5 gallon waterer and 20 chickens. The 5 gallons would probably last several days in a pinch, however I think it is important for them to have fresh, clean cool water available. My waterer is in the shade, but it still gets a little slime and a little algae if I do not clean it out regularly. I have a plastic scrubby hanging on a nail near the hose outlet. I simply dump the water, give it a good scrub and a couple of rinses and then refill with my clean cold well water. I do it first thing in the AM when I let the chickens out of their coop. That way they are eager to get on with checking for new "good eats" in the garden and other chicken fun things and are not under my feet trying to help. I fill the feeder as necessary and the morning chores for the chickens takes only about 10 minutes.
Pam in ID