No one knows for sure, only accelerated tests have been done which may or may not represent anything. Personally, I doubt Imation discs can last 100 minutes, much less 100 years, but that's just my crappy experiences with their media. It's generally accepted that "good" media is good media, like Taiyo Yuden discs... if you start out with an excellent, low error burn, then even if errors start creeping in, at least you're still ahead of the game. If you burn cheap/crappy media that's borderline at the time of burning, any errors will knock it over to "unusable".
Since recordable DVDs have only been around ~5 years, your guess is as good as any.
Of course, keep in mind, the odds of having a machine that can play back a standard DVD-R in 100 years is slim and none. Do you still have a gramophone?
"Pressed" silver DVDs are physically manfactured by stampers. DVD+-R media is organic dye. Very different technologies. Again, this stuff's been around for under a decade so no one knows WHAT realistic lifespans really are.