Alex:
 
I suggest that you speak with the Environment & Deterioration lab at
the Canadian Conservation Institute (CCI) (613 -998-3721).  My
understanding is that they have sleeves which are close to 20 years
old and are still effective.  They do tend to yellow over time (as
they are absorbing UV light) but this should only help filter out more UV.
 
Sue Maltby, Conservator