Dave, over Memorial day weekend I had my Engel plugged into my GMC truck with the stock battery, 2005 model. I had it running from Friday 11 am till Monday about 1 pm without ever starting the truck. The cooler was set to 2 +- and the bottom stuff was frozen and top nice and cold, that is what we wanted.
Truck started right up with no problems. I did not take any measurements just a real world test. During CM I had the cooler in my 4Runner running 24/7 with a marine battery, can''t remember the name but it is the high end checkers model. I drove the truck during the day and sat at night, also no problem.
take it how you want to but I think with a good battery you can run it for 3-4 days easily. Of course this all depends on your battery and if you have any leaks.