It breaks down like any material. The easiest way to sample this would be the REALLY cheap cd-r discs you could get in the late 90s. If you left one of those sitting on your dash, they would borderline melt in the sun... and for sure start peeling off the plastic if they sat on your dash for a couple hours.

Same thing with newer media - if you keep it in a climate controlled environment the sky is the limit. If you abuse it, it's not going to see anywhere near the quoted life.