I think they use about 3 amps when running. Running time will depend on the temp outside and the thermastat setting and also how often you open the fridge. What will be more important about the battery is the amp/hrs. This should be on the battery somewhere. Say if it is a 70 amp/hr battery and the fridge pulls 3 amps then theroreticly it should run for 23 hours straight. But if it cycles on and off for about half the time you will get 2 days. I have a 40 lt engel and about 2 days from a deep cycle battery is about all you can get realisticly. I just make sure i take the car for a bit of a run each day.
Aaron