How many minutes of shower is a gallon?

The number of minutes you can shower with a gallon of water depends on the flow rate of your showerhead. Standard showerheads typically flow at about 2.5 gallons per minute, meaning a gallon would last approximately 24 seconds. If you have a low-flow showerhead, which may use around 1.5 gallons per minute, a gallon would last about 40 seconds.