Depending upon humidity, my water consumption has varied from around a liter a day to well over a gallon. I would say a gallon a day would be about right if it fairly humid and one is not exercising vigorously. This does not count the water ingested with food, and does not consider body size and other variables.

Speaking as an old desert rat, I hold that the use of water for any purpose other than consumption when the supply is restricted is foolhardy. Forget bathing and washing.

Just as you can never be too rich or too skinny, you can never have too much water, well hardly ever (floods excepted).