There's no way to give an accurate answer to this question since the amount of propane you will use per month will depend on the size of the house, how warm you keep the house, how insulated the house is and how cold the winters are.
But to give an estimate on propane usage per month most people report that they use on average of 250 Gallons of propane per month for heating there home.
Sometimes the propane usage can go up to 300 gallons per month if the winter is very cold.
Expect though to use at least 250 gallons of propane in a month during winter heating season so the 500 gallons of propane should last you around 2 months.