Fridge and propane

The OP has still not replied to tell us what size and what type of fridge, BOTH of which are critical to determining just how long any gas absorption fridge might run in the gas mode. For example, one of the most commonly found gas absorption DSI fridges found in travel trailers these days is the Dometic DM2652 which is rated to consume 1501.28 BTU/hr. IF the flame actually burned constantly then in 24 hrs it would consume ~ 36,000 BTU/24 hrs and since a 20 lb tank of propane is worth about 420,000 BTU this fridge in theory would run for just 11.6 days. HOWEVER, the flame in a DSI fridge does NOT run constantly but cycles on/off as necessary to maintain proper interior temps, in which case the number of hours / days this fridge would continue to run depends on a number of variables, from exterior temperature, whether the trailer in which it's mounted is sitting in direct sun or is shaded, to how often the doors are opened, whether it's equipped with additional cooling fans, etc, ALL of which will determine how often the fridge flame cycles on/off and therefore how much gas it will consume.

In comparison, another popular fridge at the other end of the spectrum is the Norcold 323 manual light 3-way fridge that's often installed in popup trailers. Since I owned one I know it consumes ~ 625 BTU/hr but since this is not a DSI fridge, has no electronics, and the flame runs 24/7 it would require an average 15,000 BTU in 24 hrs, regardless of exterior influences. Since a 20 lb tank is worth ~ 420,000 BTU that means this fridge would run continuously for ~ 28 days even though it's flame is burning constantly. Since it has no electronics to vary it's consumption that rate will be fixed regardless of exterior temps, etc - rather it's performance will falter in extremely high temperatures.

Bottom line - unless OP specifies the type & size of fridge there's just no way for anyone to even remotely estimate how long a fridge may run on any given size of propane tank. ;)