I think how often you air it out would depend on 
the materials used to frame the cel.  I just
checked a cel I have framed like three years
ago that I never took and when I took it out it
was perfectly fine.  I am glad it was fine because
it was the first cel I ever bought and I paid
a fortune to get all my cels professionally framed
buy a place that had experience with cels (which
has since changed ownership!)