How long will 2x20 lb propane tanks last?

Twice as long as 2-20# tanks.

There are SOOOOO many variables here that the question can bring only meaningless replies. How cold is it outside? How warm do you like it inside? How large is your rig (i.e., surface area)? How well insulated? How big is your refer? How big is the generator? How long do you run said generator each day? How many watt-hours of 12 volt power do you use each day for lights, TV, radio, sound system, coffee maker, microwave, etc.?

One person/family/rig might make 'em last for many months, another could burn through them inside a week.

For everything but the generator I would expect 80 lbs. of propane to last a couple of months or longer. The generator is going to use at least 0.4 lb. per hour of operation or 40 lbs. with 100 hours of use.

Without a generator we take no special efforts to conserve propane and use it for the fridge, for cooking, for the hot water heater, and for the furnace. We use less than 6 lbs. per week whether it is in subfreezing weather (use furnace more and fridge takes less) or in the heat of summer (higher demand for fridge).

We carry two 20# tanks and have never exhausted all the propane in the first tank on any of our 3 week long trips.

The propane consumption is why I would never installed a propane generator in our camper and would go with a gas powered one instead. Much easier to get more gas at the next fuel stop for the truck.

Pretty much a trick question but we only run our heater for 5 - 10 minutes in the morning when we get dressed. I could go weeks on my 2 - 20# tanks. My generator is gas so it doesn't draw from the house propane.

40 pounders! Mighty big bottles for a TC.

You can come pretty close to an estimate by adding up the BTU requirements of the various users and doing the math. FYI, there are 21,600 BTUs in a pound of propane.