As we've all seen, plastics have lifetimes of anywhere between "eternity" (Plastics are Forever )and zero (?!?).
We certainly will see increasing amounts of biodegradable and compostable materials in the coming decades, and as every engineer will immediately recognize, the challenge is to ensure that the degradation does not occur during the intended lifetime of the product . This is very comparable to the the Deborah number in that we need to compare the degradation time to the product lifetime. (You will recall that the Deborah number is the ratio of the relaxation time to the observation time.) How about we call it the Deborah Degradation Number?
It obviously can't be listed on a spec sheet anymore than a Deborah number can be, but still, the concept needs to be understood and better communicated, don't you think?
 Sure they are forever. That's why they keep failing and breaking down into the ever small pieces that fill the various oceanic garbage patches and whale burps.
 Sadly, this is a challenge for too many engineers even today in a world of nondegradable polymers. (Notice that I said engineers, and not the materials themselves?)