I had a nice 4 layer Z Gundam cel with background that I
had framed for 5 years without any trouble.  Then my wife and I decided that we didn't like the curtains we had 
hanging in the room containing the cel, so we took them 
down for about 2 months while we were picking out new ones,
which let in plenty of sunlight that didn't used to come 
in.  When I looked at the cel after the two month interval,
the top layer had almost completely faded, and the 2nd cel 
layer was starting to go.

The cel wasn't an expensive one - but it was my first cel (purchased from Anime Link when it used to be called Anime Wink and they had a store in Venice, CA).