It can be used as long as it's still viable. You can try it by making a starter wort of 1/2 cup DME to 1 pint of water- boil for 10 minutes and then cool. Once cool, pour it into a sanitized container (a growler works great for this) and then add the yeast to it. Cover it with sanitized foil, and shake it several times a day. Sometimes you won't see any sign of fermentation, but that doesn't mean it's not working.
If I'm still unsure that the yeast is ok, after about 4 days, I'll check the SG of that starter. If it's below 1.018 or so, then I know that it fermented. You should see more yeast laying on the bottom, too. If that's the case, the yeast is fine.