A good rule might be one flush per person per day. Reduce the number of showers or baths. A good rule might be one bath or shower every other day per person. Don't use the dishwasher or garbage disposal. Don't do laundry, or take it to a laundromat if possible.
Common sense is the key to reducing water use in the house.
Remember, the drainfield was designed to infiltrate the amount of water normally discharged from the house. When additional water from rain, snow or flooding is added to the drainfield, its ability to handle household water becomes seriously limited. April 1997 NDSU Extension Service, North Dakota State University of Agriculture and Applied Science, and U.S. Department of Agriculture cooperating. Sharon D. Anderson, Director, Fargo, North Dakota. Distributed in furtherance of the Acts of Congress of May 8 and June 30, 1914.