It's going to depend on how you use it, how much water it holds, how hot you want it and what the water temp is when you start. 8 BTU raises a gallon of water one degree. If your spa is 500 gallons, in a perfect world, to raise the water from 80 degrees F to 104 degrees would take (104-80) X 8 BTU X 500 gallons = 96,000 BTU. That would be about a little more than a gallon of propane. Heating a spa is pretty wasteful, but I would guess you could heat it with 2 gallons.