I've mentioned this before, but I have had 
several Disney and Warner Brothers cels hanging 
on my walls since the mid-1980s.  Not a trace of 
line fade.  

On the other hand, the lines on some of the anime 
cels (all from AIC and Toei) I framed using the 
same materials started fading after about a 
year.  Light appears to play some role in the 
process, as similar cels stored in celbooks have 
not faded.  This suggests to me that there is 
some difference in:

the xerographic process among the different 
companies,
the composition of the paints, or
both of the above.  

I, too have noticed that lines touching warmer 
colored paints (typically browns and yellows) 
seem most likely to fade, although ocasionally 
blue tones are a problem.  I really miss having 
the anime cels on my walls....