It depends on two things. How big the hot water heater is, and do you have a standard flow or low flow showerhead.
A hotwater heater only delivers about 70% of its capacity as hot water. So if you have a 30 gallon heater, you get 21 gallons of hot water. If its 40 gallons, you get 28 hot.
A standard flow showerhead consumes about 2.5 to 3.5 gallons per minute, so you should get a 6 to 8 and a half minute shower for a 30 gallon heater. For 40 gallons, it is 8 to 11 minutes.
Low flow shower heads consume 1.2 to 2.0 gpm, so consume about half as much hot water, i.e. twice as long of a shower.