Opened packages of dry yeast (active dry or instant) can be stored in the refrigerator for three to four months. If your yeast is older than this, it may still be good. Proof your yeast to find out if it's still active by adding 1 teaspoon of sugar and 2-1/4 teaspoons of yeast (one envelope) to 1/4 cup of warm water. Then, wait 10 minutes. If the mixture bubbles and develops a yeasty aroma, the yeast is still good.
Tip: Want to store yeast longer? Stick it in the freezer, and it will be good for years.