About 45 million years actually.....
45 million year old yeast ferments amber ale
Seriously though, Bobby M recently did a test on year old stored yeast here; http://www.homebrewtalk.com/f13/test...bility-126707/
And my LHBS cells outdated tubes and packs of yeast dirt cheap 2-3 dollars each
and I usually grab a couple tubes of belgian or other interesting yeast when I am there and shove it in my fridge. and I have never had a problem with one of those tubes.
I usually make a starter but I once pitched a year old tube of Belgian High Gravity yeast directly into a 2.5 gallon batch of a Belgian Dark Strong, and after about 4 days it took off beautifully.
You just need first to apply the "sniff test" if it smell bad, especially if it smells like week old gorilla poop in a diaper left on the side of the road in the heat of summer.
Then make a starter, and if it takes off you are fine.
THe only issue I have is, did you sanitize the tube you put the yeast back into? Most of us store in mason jar that we either boil or sanitize with starsan or iodophor. If your vessel wasn't sanitized, then that may be an issue, and you'll know with a sniff test down the line.