not long. Depends on the amperage the heater draws and what kind of batts yo have. In most care maybe a few hours. In a truck a bit longer
a good working alternator and good battery will last 2 hours. running headlights. maybe longer with today's higher amp batteries. my last motorcycle went an hour. but those are smaller amps. i did a drive from pocatello, id. to salt lake, ut. with the alternator unplugged. 170 miles at 55 mph. headlights on. only got 140 miles before the car started sputtering. plugged in the alternator and went home. 55 was the speed limit back then. and i don't think 12v heaters existed. but i'll see if i can find a link on the internet to back that information up.
