Hello
I did an experiment with a 'trash' cel. It had good lines 
to start out with and I put it into a place where direct 
sunlight would fall on it for at least a certain amount 
of time a day.
In less than one week the lines were virtually gone.
Scary, eh?
As Yann says, there is always some UV in a room, and if 
you have cels framed, you're always running the risk of 
line fading. If what you're interested in is preservation,
you should keep your cels in a hermetically sealed room 
with carefully controlled temperature and humidity and 
zero light. However, it's hard to appreciate your cels like 
that. So, in my mind you have to strike a balance between 
conservation and appreciation. Which is one of the reasons 
I love having a cels webpage. It allows me to look at the cels
whenever I want without dragging out my celbook and exposing
my precious cels to light while I drool on them.
Mata ne!
Ryochan
(Who wonders about the effects of human drool on cel acetate)