Step 1: Check to see if you have a low-flow showerhead
Anything less than 2.5 gpm (gallons per minute) counts. There should be a label on the showerhead. If you can't find one, you can always rely on the saucepan method. Take a saucepan of known volume (4 quarts to a gallon), and run it under the shower (on cold!) to see how many times it fills up in a minute.