According to Kurt Gerecke, a storage expert and physicist working for IBM in Europe, a standard burned CD you pick up off the shelf is going to last you only two years, five years at the upper end of the spectrum for quality media. The CD supposedly breaks down over time, and anyone looking for a serious backup solution should consider using magnetic tape due to its longer lifespan.

"Unlike pressed original CDs, burned CDs have a relatively short life span of between two to five years, depending on the quality of the CD," Gerecke said in an interview this week. "There are a few things you can do to extend the life of a burned CD, like keeping the disc in a cool, dark space, but not a whole lot more."
While it's true that a standard set of tapes, stored properly, is going to last you a very long time, it's possible there may be something else behind these statements. The dye in CDs does degrade over time, but often I make use of burned CDs that are well over 8 years old, still functioning. The quality of the discs you buy and the method you use to store the discs will impact the life of the CD. He also does mention the use of hard drives as a backup-worthy medium, with bearings wearing out being the primary concern. Then again, if you are using hard drives as a backup solution, the hard drives should be stored in a safe place, not in used, and thus not being worn down. Gerecke's advice seems more applicable to businesses, though, and for just about every desktop user out there, CDs, DVDs and, at most, hard drives, are likely more than enough.