The type of shower system, and especially the showerhead, you have installed will determine how much water a 10-minute shower will use.
If you have a low-flow showerhead installed, you can expect to use about two gallons of water per minute, equalling 20 gallons throughout a 10-minute shower.
With a standard showerhead, around half a gallon more water will emerge each minute, so a 10-minute shower would use somewhere close to 25 gallons.
*1 gallon = 4.54 litres.
