A big issue in longevity of transfers is UV damage. (I've got window-sill
tests to prove it :)
Kathleen Carr's new book "Polaroid Transfers" (which came out this year I
think) states that transfers can keep for years if they have UV protection,
and are kept out of direct sunlight. Her book (which is excellent, btw)
quotes someone from Polaroid on the matter. And she lists some brands of UV
coatings. You could use UV glass too, when framing a piece.
Hope this answers your question more specifically.
Karen Molloy
kmolloy@tiac.net