The average American spends about 8 minutes in the shower each day--which shakes out to be about 17-18 gallons of water per person, per day. *This doesn't include the portion of time where the water is left to run until it 'warms up.'
As a native Midwesterner, this was never an issue that caused me great concern, until I moved west. Reducing the length of time water is running in the shower by one to two minutes, even, is a difference of 14-28 gallons per week per person (42 gallons/week fewer than average for the rock stars that keep their shower to 5 minutes)!
On a daily basis, this really adds up--particularly in regions of the country where droughts, agricultural needs, and debates over water rights are becoming more and more concerning. Your city water provider would surely be happy to contribute a wealth of resources when it comes to using water more conscnientiously. If not, Denver Water has one of the best advocacy campaigns in the nation, in case you were wondering!