It really depends on the unit / construction. Our 2nd floor apartment is 1000sqft with natural gas heat, newer construction, newer windows, energy efficient appliances, etc. We literally leave the thermostat at 72 all year round. Never had a bill over $200 in summer or winter. We stay quite insulated.

Our previous place was on the top-floor of creaky triple-decker with old-ass appliances, drafty windows, etc. It was a downright struggle to keep it at 68. It seemed like the heat would blast non-stop and it was still cold.