There are cels from Snow White, Fantasia, and 
other series that have held up well for over 60 
years. Many cels haven't aged that well, but they 
probably weren't treated as well. Lots on ebay. 

I have a few cels that are at least several 
decades old, and they seem to have held up pretty 
well. For newer cels with xeroxed lines, I'd 
assume the lines would need restoration in the 
future, but it probably won't matter by then.