If it's been kept refrigerated, but not frozen, you're looking at a couple of months. Freezing yeast kills it. Try and get some nutrient, some liquid yeasts come with some, which will wake up the yeast a bit more.
I'd be hesitant to use it though just incase it isn't active. I wouldn't want my time to be wasted.
I suppose you could get a wee cup of warm water, throw some sugar in and a small bit of your yeast. You'll quickly know if it's alive or not.
Is this for your root beer? Will you post some pictures?