I've used a Wyeast pack over 8 months old with good results. I had the pack refrigerated, so that helped it I'm sure. It never appeared to do much of anything when I made a starter, but I tasted it and fermented out. I'd say it took over a week. I would have made a 2nd starter, but ran out of time. The yeast did so well it came out the airlock with a healthy blowoff. It climbed out of a 6.5 gallon bucket on a 5 gallon batch. I'd recommend giving it a few days and then checking the gravity or giving it a taste test. That yeast may surprise you yet.