How long will propane
last? Hard to say. First of all, propane
in the tank lasts forever. It doesn't go bad.
is nearly impossible to figure out, in practical terms. Here are some guidelines you can use:
1 pound of propane has 21,600 BTU's....1 gallon of propane has about 91,000 BTU's, and a cubic foot of propane has 2570 BTU's. A cu ft of natural gas, btw, has a bit less than half that, around 1075 btu's (it varies), but the condensation
that results lowers the heat output by around 10%.
So, if you have a (say) 8,000 BTU burner (which is the size of a large gas burner on your average home range) and run it full out for an hour, you will use about 1/3 of a pound (0.37 and change, actually). However, people rarely use a stove full out.
If you had CNG (natural gas) and switched to propane, you should be getting around 2.5 times as long between fill ups, if your use remains the same. A good reason (I think) to switch to propane, but propane is heavier than air and natural gas isn't so propane requires special handling.