I was just making pizza dough and noticed that I am down to about a 1/4 cup of this yeast.  Then it occured to me that I have been using this same package for TEN YEARS.  It is still vital, having been kept in doulbe ziplock freezer bags all this time.  It has even survived a move.  Is it a good thing to keep it around this long?- I don't know.  I can only tell you that it still works!  Keeping it dry and in the freezer must be the trick.