It depends how you treat it. I don't see any reason why a CD shouldn't last forever if you look after it. (and that means not having your mates back after the pub every weekend listening to all your CD's all night, and leaving them scattered on the floor round your stereo, not in their cases!)

CD's should last a lifetime if they are stored in a cool dark place. However, beware of CD-R's and CD-RW's -- they are different and can become unreadable within 2 years (http://computerworld.com/hardwaretopics/storage/story/0,10801,107607,00.html).

CD-R's and CD-RW's have a layer of writable medium, thin and metallic in nature. It does degrade over time. For important storage "archival quality" cd's and "gold" cd's purport to have an extended lifetime. They are, of course, more expensive. Heat and physical abuse are the worst enemies of cd's. Scratches on the clear side look bad and effect performance, but damage to the decorated side is even worse. Scratch this layer a bit and the laser light passes thru and is not reflected to the light sensor. thus no picture/sound data is being read.

life expectuncy is around 10 years for a top quality medium as long as you treat it with as much care as possible.

Your milage may vary depending on how often you play it, and how you store it/handle it. One thing I would recommend is not leaving a player on PAUSE for a long period of time-if you're going to need to stop it for more than, say, 5 minutes, press STOP instead of PAUSE. Don't do a lot of "play-seeking"- that is, holding the FF or REW keys down in such a way that the music still plays as it's being rewound/fast-forwarded. Do "track skips" by briefly pressing the FF/REW keys to get to the track you want instead of "play-seeking". Ideally, CDs should be kept in the jewel cases or CD "trapper keepers". If you need to quickly switch disks and don't want to fuss with the cases right this moment or you don't have enough cases for all of your CDs, put it "label side" down. The shiny side is what is actually read, and that's where scratches may cause dropouts or other problems. I would use a CD until the skips or dropouts get annoying, and they can't be solved by cleaning of the CD. If there are really stubborn spots, try actually washing the disk under cold water, and then wiping it while it's still wet. Of course, you don't want to play it until it's dry. In other words, use it until it shows it's age.

For music CDs, don't do too much "play seeking". Do "track skipping" instead. Also, leaving it on pause for a long period of time can also cause exxessive wear. Playing from start to end on a music CD without pausing or skipping tracks or anything is probably best for it, but don't lose too much sleep over that. Just don't get too "FF/REW happy" :) Store CDs in some sort of case. If you don't have the case any more you can get CD "trapper keepers". When you just want to set one down, put it label side down. The shiny side is the part you don't want to scratch. I would use a CD until the skips become annoying and they can't be remedied by cleaning. So, bottom line-use it until it shows it's age. I would say about 10 to 20 years if taken care of and played on a regular basis. There may be places where you can recycle old CDs that no longer work. If it still works, however, you may be able to get some moolah for it at a CD store.