An unused cartridge lasts 10 years if properly sealed. Store in a cool, dry location. Between uses, if you care for them properly, your Purifier cartridges will last up to three years after first use.
The one part of the technology that will degrade is the Activated Carbon (which filters heavy metals and chemicals). Activated Carbon absorbs impurities from the air, so it naturally degrades over time once unpackaged. You can limit this effect by drying thoroughly, then placing it into an airtight bag.