a 30 pound tank contains enough propane to supply 646,440 BTU of heat. If the furnace consumes 42,000 BTU per hour of operation, that would give you about 15.4 hours of furnace operation. Of course you don’t normally have the furnace running continuously. So if it runs for 10 minutes of every 30 at night, and 5 minutes of 30 during the day, the tank might give you about 2.5 days of operation.