Longest I've refrigerated Wyeast was about three months and it worked great. I always take the pouch out of the fridge the night before I brew and allow it to warm up to room temp overnight. I then smack the pack first thing in the morning and it's always ready to go by that evening.
If you're worried about viability you can always make a starter and just seal/store the results back in the fridge after it's fermentation cycle ends.....this way you're "reviving" and refreshing the yeast.