According to this article from the Australian paper the Sydney Morning Herald, there is already evidence of DVDs starting degrade within a mere 2 years of their original manufacture date.

Although the evidence is so far a bit sparse, indicators are that between 1 and 10% of all DVDs may have some kind of corrosion problem. Of the early potential culprits, flawed cases seem to be a forerunner for the cause of the problem, with complainants believing that certain glues in the cases may cause the protective plastic layers of the discs to degrade prematurely.