Length of viability of fern spores

[roachie at NOSPAMroach.org]

I'm wondering how long fern spores remain viable if kept in a dry 
place? I know there are various societies that trade and sell spores,
so they have to be good for at least a few months, but how long beyond
that?

I have a Athyrium Goeringianum 'Pictum' (sold to me as a Japanese Painted
Fern) that I'd like to propagate, but I'm unable to do it immediately and
I'd like to know how long I can safely hold onto the spores.

Thanks for any help.

Lyn