In my experience of over 25 years, an airtight glass jar kept in a dark area is perfect for up to a year. However, you will start to lose potency around 10 months to a year as long as the humidity in the jar doesnt drop below 60%. If the humidity drops below 60% they will dry out to much and crumble on contact plus lose potency quicker if the humidity is to low. I personally use hygrometers in each one of my jars so that I can keep an eye on the humidity as well.