We get one therm (= 100,000 BTU) from every 1.1 gallons of propane. A standard 5 gallon tank holds roughly 4.7 gallons. So a full tank would hold 427,000 BTUs. Therefore, a 40,000 BTU propane heater, such as SunGlo or Patio Comfort on high can get 10 hours and 40 minutes out of a tank. A 46,000 BTU EvenGLO on high can get 9 hours and 15 minutes out of a tank. Pyramid heaters have input ratings of up to 66,000 BTUs (to make up for some of their heat deficiencies) so a tank of propane would only last 6 hours and thirty minutes. Sometimes exchange tanks are not brim full. I have been told that some exchange tanks contain only 15 lbs. of propane. You will know this by weighing it. 4.7 gallons of propane weighs about 19.9 lbs. The tare on an empty propane cylinder is approximately 17 lbs. So, if you buy a tank of propane it should weigh about 37 lbs if it is full to capacity. Buying propane at a refill station is usually a better deal, but may be more time consuming.