To determine the shower length, you must investigate the water heater’s first-hour rating or FHR. This number refers to the amount of hot water that the water heater can produce in running for the first hour. It is essential to understand that the first-hour rating assumes that the tank portion of the water heater is full. The higher the FHR on a specific unit, the longer your showers will last. The average shower uses 2.1 gallons of hot water per minute. To determine the exact amount of time you will be able to shower comfortably, divide the first-hour rating by 2.1.