We "dry" flush liquids and then do a rinse with a pint or so of fresh water at the end of the day. Solids take a bit more water but never as much as a half gallon. We had a 6 gallon tank for years and found it was good for about 2 days for the two of us. About 5 years ago I upgraded to a 13 gallon tank and we can easily go for 5 days without pumping out.