I know Amazon says they have 11 9s, but merely given that there have been 5 mass extinction events in the past 500M years, you probably can't do better than 8 9s. (11 9s gives you even odds that your data will last 70 billion years, as long as you pay Amazon's fee.)
You're probably right that Amazon holds your data for as long as that, however, there's another way to look at 11 nines: If you upload one hundred billion (10^11) files on Glacier, you expect to lose one of them per year, and not more. That's probably realistically achievable.
Compare this with S3, where you will lose 1 in one billion files per year. Shocking!
