To know with accuracy how long a full tank will last:
First, look up the BTUH rating of your furnace.
A gallon of propane contains 95,000 BTU. Divide that by the BTUH rating of your furnace to determine how long a gallon of fuel will allow it to run for at full capacity.
Determine the capacity of your tank. A typical 40-pound portable tank can store up to 9.5 gallons of actual fuel.
Multiply propane burn rate by capacity to find out how long you can expect a single tank to last.