I don't use CD-RWs, but I do know that the older Memorex discs (with the blue dye) lasted for me, about 4-5 years. They're all unreadable now. The interesting thing is, several of those CD-Rs were never played and were, litterally, kept in the dark all those years.
I pulled one out about a year ago noticed the label side was yellowing, so I tried to play it and forget it.
This only happened with the blue dyed discs.
I wrote Memorex complaining about this and those spineless people didn't reply to me at all.
My Kodak disc of about 8 years, is still playable.