Re: 5 gallon propane tanks
We swap out propane tanks less than once a season at our homestead. This is every other weekend use over the course of the season. Used just for cooking, the propane actually lasts a looooong time! If you start trying to heat your cabin with it though, expect it to last just 3-5 days, depending on your weather. I wouldn't waste propane on cabin heating unless you have nothing else. High quality fuel like propane is best for stovetop cooking. Gary gave some nice calculations about use. I'll remember those.
Here's one way to know how much propane you have in the tank. Weigh your empty tank on a bathroom scale. Weigh it again after filling. The net weight change is proportional to the amount of propane IN THE TANK. Propane weighs 5lbs/gallon, so the net change should be about 25lbs. It's important to check this because an inexperienced person at the pump can screw up filling the tank. I've seen a person attach the connection incorrectly resulting in about half my propane just dumped on the ground to evaporate!
I keep an extra full tank ready for each propane appliance I have. As soon as I empty a tank I swap it out with a full one. I get cheap old tanks at the flea market for 6-7$. I then use these as trade-ons at the gas station for a new, full tank. It's a good way to get old, potentially dangerous tanks off the streets, and it saves me the cost of paying for the new tank.