Week 2 - The Muddiest Point

I don't know if this really counts as a "muddy" point, but I am curious about how the manufacturers know how long their media will last, as listed in the slide about "Media Life Expectancies" (slide 56). I especially wonder this about Microfilm, with its potential life span of 500 years - the media is not yet that old, so I wonder what sort of tests they've done that let them know it could last that long.