We have a three gallon waterer in our run, which seems to last our flock of 23 young hens about 3-4 days.
We also keep a one gallon waterer in the coop, changed out every day.
The three gallon waterer in the run, though, tends to get really muddy and gunky with the girls everyday activities. The best thing we've found to keep it fresh and clean is to lift it out of the run every day, swirl the water in the reservoir lip around a couple of times, and dump it out on the ground, allowing it to refill with clean water.
It takes just a bit of practice to swirl the water around in the lip enough to wash the gunk out each day without splashing too much fresh water, but once you get the hang of it, it works really well to keep the water fresh and clear in between total wash and refills.