A 20-pound propane gas tank will last between 5 and 8 hours with a 50,000 BTU fire pit. That's about two or three uses depending upon how long you keep your fire pit going. Keep in mind that each 20-pound tank costs about $50, while a 100-pound tank will cost about $120. It's much more cost-effective to buy larger propane tanks if you have a safe storage place for them near your fire pit and a long enough hose. It's much more convenient to buy the 20-pound propane gas tanks and get a fire pit that has storage underneath it for the tank.