A fairly recent discussion of the stability of data storage media appeared (within the last year?) in "PC Magazine." The consensus of media manufacturers was that data stored on CDs - archived and not in constant use - can be expected to be well over 20 years, and perhaps the CD data could last longer than we will.
An important factor is that an ARCHIVE set should be maintained in proper storage, and would be used only for the occasional production of the MASTER set(s), which would be used in turn for the production of the media used for the day-to-day distribution of data (the DISTRIBUTION sets).
Since CDs for mass data storage haven't been around that long, there is a degree of uncertainty, but some testing has been done (accelerated aging via temperature cycling, etc.) that lends a certain credibility to the prediction.
When film is used in the same way, we know that it CAN last more than 20 years, but a lot of the old film hasn't stood up well. Pulling film more than twenty or thirty years old out of archives involves more of a restoration - now usually done digitally - than just a copying.

The more important question is, when any stored information is pulled out of archive after 20 years or more, will we have a machine that can read it.
There are (or were) tons of punch cards archived from the 40s and later that are essentially unreadable, because nobody makes pcard readers anymore. I'm sure there are large warehouses full of reel-to-reel digital tape, backups from the ancient dinosaur mainframes, that may (or may not - remember Schrödinger and his cat) have perfectly preserved data on them, but noone will ever try to read it because there are no machines left to do it on.
As new storage media are developed, old ones drop into disuse, and the machinery to read them disappears. Does anyone remember the old 8 inch floppys that "modern" word processors used in the 80s. You could still buy them in 1990 - but can you find a drive now? Anybody tried to read the 5.25 inch floppies that were common in 1994 or so? Anyone tried to buy a NEW turntable that will play 78 rpms?

A few years ago a well-known hard drive mfr advertised (on April Fool's Day) the startling new, incredibly fast "Write Only Drive" - with virtually unlimited capacity, and at very low cost. The few thousand people who contacted them for more information were probably disappointed, but frankly any idea that there is a way to put history away forever probably requires the use of the WOD.
If history isn't alive enough to be occasionally dusted off and cleaned up, it might as well be filed on the Write Only ...(?)

Long rant, but I feel better.
I haven't checked whether the review is still up at the PCMagazine.com site, and I don't recall exactly when it was published, but some of their stuff is available at PCMag for anyone who might wish to look.

John