Response from Dr. Clayton Cone:
Active Dry Yeast, at <5% moisture, is originally packaged in an oxygen free atmosphere either via nitrogen flush or vacuum. Under these conditions they will loose about 20% activity / year when stored at 20C.(68F) and about 5% activity / year when stored at 4C.(40F). It is the presence of Oxygen and the pick up of small amounts of moisture that causes the yeast to deteriorate at a faster rate, once the package has been opened. If you can vacuum pack (kitchen vacuum package equipment)or store in an air tight container and refrigerate you may retain a substantial amount of the activity. It would be wise to increase the inoculum 50% to be on the safe side. It is always a gamble, depending on how much moisture the cells have picked up each time you open and close the package and return to the refrigerator.