it will mainly depend on what size battery you have in your vehicle.
this means it will use "less than" 0.3Amps per hour.
If you have a 600 amp battery (if it was charged to peak, and in A1 condition) it would theoretically last 2000 hours until it drains the battery flat.
How long you can leave it on before the battery wont start your car is another thing. How much amperage is required to start your car? and bare in mind normal Lead-Acid batteries (like in most cars) arent designed to be ran flat, recharged, ran flat, etc etc. They are there too handle quick heavy loads (like starting a car) and then being recharged by the alternator thru trickle charge. I have 2 x 600 CCA batteries in my 4wd, with an invertor that has a <0.5A rating in idle mode and its been fine for a week with the PC in hibernate, but leave the laptop on full time, with the screen, my car wont start a day or so later, due to my 4wd's huge starting requirements.
most small cars require about 250amps to start them, and if you have a 400 amp battery, expect it too output about 300-350 after some use, so, really, its hard too say how long will it last...