A 500 lb propane tank actually holds around 420 lbs of propane when you factor in the room in the tank for expansion.
So if you have a 500 lb propane tank your 500 lb propane tank will hold 420 lbs of propane which is 96 gallons of propane.
So you would have 96 gallons of propane in your 500 lb propane tank.
Now for a 500 gallon propane tank you would have 400 gallons of propane which usually lasts either 1 month to 2 months during the winter depending on the size of your home and how cold it is.