There's approximately 91,500 btu's in a gallon of propane. I have a 12 cu/ft Norcold refrigerator that burns 2400 btu/hr. So one gallon of propane will last about 38 hrs. However that's if the refrigerator runs constantly. Again, assuming it runs for 8 hours out of a 24 hr. period a gallon of propane should last about 5 days. Naturally it all depends on how hot the outside temperature is, how full the refrigerator is, the temperature setting and how often you open the door. Using these #'s, a 20lb tank would last ~20 days.