For an absolute answer on how much propane your furnace (or any other gas appliance) uses, look at the BTU rating on it's nameplate. In this case, BTUs are used to measure the amount of heat energy in the gas it's consuming, not the heat output of the unit.
There are about 90,000 BTUs in a gallon of propane. If your furnace is rated at 30,000 BTUs it will use 1/3 gallon of propane for every hour it's in use. (30,000 / 90,000 = 1/3)
If the furnace is running continuously, you'll use 8 gallons of propane per 24 hour period.
If the furnace is running an average of 50% of the time you'll use 4 gallons every 24 hours.
And so on.