An adult hen drinks about 0.05 to 0.08 gallon a day in cool weather and up to 0.16 gallon in hot weather, per Utah State University Extension. So 6 hens can empty a 5 gallon waterer in about 5 hot days.
How we count days of water
Utah State University Extension says "in cool weather each adult chicken will consume about 0.05 to 0.08 gallon per day; in hot weather, 0.08 to 0.16 gallon per day." We divide each waterer's stated capacity by 0.96 gallon, what 6 hens can drink on a hot day, and round down. In cool weather it lasts about twice as long.