What is the average expected lifespan of a 20 cubic foot chest freezer?

Really depends on how and where it is used.
20-30 years is not unheard of.

Thank you. I'm asking as our 20 cu.ft. unit is approaching its 40th year and I am getting concerned about the possibility of it quitting sometime when we are away.

Make sure your household insurance covers the contents, then relax..
I'd be inclined to start looking for a replacement though..

Our was 30 yrs old when I sold it on Craigslist.

Over 50-60 years is normal. If the wires rot or the start relay fails these parts can be replaced, but the old R-12 freezers and refrigerators are much, much more efficient and longer lasting than the new ones with R-134a that the liberals forced down our throats without anyone realizing that a gas 5 times heavier than air cannot rise into the ionosphere and deplete the ozone, which is a scam anyway.