We use propane for cooking. Pretoriens have a built in propane locker in an aft lazarette on the port side. Propane needs a safe place to vent outside the boat so any leaks don't settle and, when sparked, go BOOM. Our locker is fiberglass in, separated from the rest of the boat and vented at the stern.
We go through our 20lb tank of propane in approximately 2.5 months. The length of time we go changes depending on how much we eat out, and how cold it is, and so will vary depending on where we cruise. Our propane use is on the heavier side in the winter when we make a gazillion cups of coffee, cocoa and tea and bake more. Some people are smart and put their extra hot water from the kettle in a thermos. We haven't internalized that trick yet.
I wondered stuff like that when we were thinking about boats and when we first moved aboard. Propane storage is tricky so when buying a new boat, how much is enough? Should we carry extra propane?
We also use the main tank for our BBQ with an adapter. As a marginally reasonable back up we carry a few small disposable BBQ tanks. So far a 20lb tank is fine for us. We can cook full time for two for approximately 2.5 months. We might add a smaller tank on our stern rail someday when we travel somewhere where we can't fill the tank easily in that timeframe.