I agree, 5-7 years at minimum, as long as they are stored correctly. I've seen the smaller propane cylinders (for plumbing torches) that were older than that, and they worked. The propane doesn't break down; it's just a matter of micro leaks in the valve over time.

I think the bottom line is: if you shake them and can feel liquid propane sloshing around, they're okay to use.

Rust, as has been mentioned, is a problem. If it's only on the threads, you can wire brush it off. It's critical that the top rim be smooth and flat, since that mates with the rubber seals on the appliance. It's also important that there be a minimum of crud on the inside of the rim, where the "nose" of the appliance pushes down and opens the valve.

I've resurrected a couple of old cylinders with wire brush, light oil and fine emery cloth. (Tightwad!) They worked fine, though I checked very carefully for serious corrosion to the cylinder walls and for leaks once mated to the appliance.

If I was storing for the long term, I would probably use a rag with light oil and lightly wipe the exposed surfaces. And I would keep the caps on. You could even wrap the entire top with electrical tape to discourage entry of damp air.