How much water does a 25 minute shower use in gallons?

Obviously it partly depends on the shower head, but most will give about 2.5 gallons per minute, so you are using about 12 gallons of water, possibly about 8 - 9 of it hot.
Place a 1 liter bottle below the shower and measure how many seconds it will require to fill it. Then:

Number of gallons in five minutes = 78,1/ Number of seconds
An average shower head emits about 1.3 -1.5 gallons per minute, so that would be 7 -8 gallons.