We have an accelerated aging experiment, and we sample the part for degradation throughout the test. We also "shelf age" another test group of polymers from the same batch by placing them on the shelf.

Every year or so (or sometimes 5 year spans), we take the shelf samples and look for degradation, and compare it to the accelerated data.

In one polymer's specific case, free radicals in the polymer chain slowly react with oxygen and that's how it breaks down (takes about 10 years if the polymer isn't stable), so our accelerated aging is to put in in a pressure vessel full of high-pressure oxygen, and apply a little heat. We can get 10 years of degradation (roughly) within 2 weeks.

This is almost certainly what the archival blu-ray format has done and is doing.