A toilet tank can hold close to 3 gallons of water; however, when flushed, it is normal operation for the toilet to use only about 1/3 of the tank water. This occurs because most of the water in the tank is used for the downward weight that gravity provides for a better, more powerful flush. This is referred to as "bulk flush."
Class Five and Aquapiston canister style flush valves stay open for roughly 1 second when the trip lever is pushed down. Flapper style flush valves stay open for almost 2 seconds.