Um, forever! I have it plugged into the cig socket. It CLAIMS that it pulls 2.1A at 12V, but I haven't measured it yet, which I can easily do and plan to. If the claims are true (they're usually over-rated), then it's pulling about 25W, which is a little less than my laptop pulls when pulling DC-DC (no inverter). Thus, I'm not sure I'd recommend using it too long without solar power or keeping a good watch on the house battery voltage.
2006 RB50 6.0 Diesel
Flatplate water heater (mounted INSIDE), portable butane stove (no propane)