.

Accordingly, is there a shelf life on propane?

Propane does not have a shelf life - you could store those tanks for another 100 years, and assuming they don't leak and the valves still work, your great great grandchildren could use that propane to grill up some steaks in 2113.

Subsequently, question is, how long is a 20 lb propane tank good for? 12 years

Beside this, how long does it take for propane to dissipate?

This process with Propane is slow and may take many weeks to be completed. The half life of Propane in air is estimated (assuming 20C) to be about 14 days.

How many years does a propane tank last?

Answer: A standard 5lb propane tank can potentially last 50 or more years with proper care and use. Typically, these types of tanks should be check for damage, leaks, etc. and re-certified for use 12 years after the date of manufacture and then every five years after that.