Federal regulations mandate new shower heads flow less than 2.5 gallons per minute with a line feed pressure of 80 pounds per square inch.
Although actual consumption varies with water pressure at the shower head inlet and the exact flow rate of the shower head, a reasonable average for a 5 minute shower using a low-flow head would be 7-8 gallons of water. This is because most homes have far less than 80psi line pressure, the typical pressure being around 50psi.
Older high-flow or unrestricted shower heads in systems with more typical 50psi water mains pressure can flow over 4 gallons per minute. A 5 minute shower would use 20 gallons of water with an older unrestricted shower head, although it could be more.