You need to realize that if we have a full hook up site, we can shower as long as the hot water lasts. The key to shower length is not the hot water as much as the holding tank. When we do not have a sewer connection and must hold our waste water for days, the length of the shower shortens so we can hold longer by using less water. We have a gas/electric hot water heater that is 10 gallons. With a flash hot water heater and a full hookup site (ie one with sewer hookup) the length of the shower is almost unlimited.