Depending on the BTU's that are used when burning off your tank is how long it will last. 1 pound of propane is 22,000 BTU's. A 20 pound tank will hold 4 gallons of propane or 366,000 BTU's. Your heat and the burn off will vary on your stove so there is no real way to answer your question truly. It will last awhile depending on how often and long you use it. An average is 6 to 7 hours of burning.