How long will 500 gallons of Propane last?

I am buying a 1238sf split level home, that is very energy efficient. Some key features are lennox G61 two stage furnace 95% with touch screen t panel, Rinnai on demand hot water heater, all exterior walls blown with R-23 dense pact optima w/ fabric, R-50 in attic, windows are low E/argon gl which meets energy star .3, zip wall system, zip roof, 3 bedrooms, 2 full bathrooms, gas stove, certified blow door tested, all duct work sealed and insulated to energy standards, spray foam all over heads, forced hot air. So my question is how long would a 500 gal propane tank last on avg in the winter months and then after the winter?