How many minutes will 40 gallons of hot water last in a shower?

That depends on both how quickly water flows out of the shower head (gallons per minute) and how hot the water is (ratio of cold water to hot water).

Also depends on the recovery rate of the water heater. As you use up the hot water, the water heater is making new hot water, but not usually as fast as you are using it. A high recovery rate will extend the time you have hot water.