The real-time stability testing indicates at least 20 months so far with no loss of potency. The original standard has been preserved in a cool, dry, light-free environment, but not vacuum sealed or refrigerated.
I believe the manufacturer insures potency for up to 24 months, based on anticipated or accelerated stability testing methods. It looks like it'll be much longer than that though. Put it in the freezer and it'll probably be good indefinitely.