Lots of things affect water usage. If they are too hot they will drink more water than normal. As a general rule, I usually emptied the waterer once a day and gave them fresh water anyways. I didn't really pay attention to how much they were using up. I was using a one quart waterer.
I used a 1 gallon plastic waterer when we were brooding our day old Banties back in May. I never really paid attention to how much they drank, just made sure that they always had clean, fresh water everday, at all times.
I usually empty, clean and refill morning,afternoon and night it just seemed that they weren't drinking much but I guess thats because I'm just replacing it 3 times a day.