Most shower heads these days limit the amount of water flow to less than 3 gallons a minute (usually 2.5 gpm). Multiply this times the amount of minutes you shower and you will have a rough estimate...
15-25 gallons Water Trivia Facts
1. Water is the only substance found on earth naturally in three forms. True (Solid, liquid and gas)
2. Does water regulate the earth's temperature? Yes (it is a...
There are Many factors to figuring out this question.
You would have to know how long the "Average Person" Spends in the shower,
The Water Pressure to the Shower (To find out the amount of water...