How long the 500 gallons of propane will last you will depend on what you're using the propane for.
If you're heating your house with propane and a propane furnace and your house is around 1,500 square feet then you can expect to use at least 250 gallons of the propane per month and maybe even 300 gallons per month depending on the climate and how cold your area gets and how well insulated your house is too.
Most 500 gallon propane tanks last for at least 2 months without needing a refill but you should have the propane tank refilled when it gets down to about half empty so you don't lose propane pressure to your appliances.
Most people use at least 250 gallons of propane per month for heating and if you use propane cook stove and propane water heater then your usage can go up.