I am no expert, but here is what I think. I have 
read posts from people o nthis board who have 
noticed that their cels faded even with minimal 
exposure to sunlight and room lights. When I 
framed my cels, I used UV glass and hung them on 
the same wall as my window, so direct light could 
not reach them. I figure the UV glass will filter 
whatever light does reach them. What I think you 
need to ask yourself is: Are my cels important 
enough to me to take a chance on framing them 
like that? I asked myself that, and when I 
figured that I am talking about framing about 
$1000 worth of cels, I said no. I have added a 
link to my website that tells what I learned 
about framing cels myself, and how I was able to 
save money and get the job done right without 
puting my cels at risk. But, as Stephen said, 
cels are tough, so we really don't know how much 
an individual cel will take. Hope this helps.