Check out this article that talks about how long various storage formats are good for. CD-R and CD-RW, depending on quality are only good for 2 – 5 years, the same with DVD-Rs. Even hard drives only have a period of a few years depening on their bearings.
This just proves what I have always said which is you should have as many backups as possible. Take my MP3 files for instance. I have them backed up on two different hard drives, have them on my iPod and backed up on CDs. I have put so much work into ripping and downloading all those files, the last thing I want to do is lose even one of them, so I back them up multiple times. You also have to take into account that storage formats change every so often, so you have to be ready to migrate your data to yet another storage medium in only a few years.
Just remember, having multiple backups of your information is never a bad idea.