They typically last about 10 years if there kept cool and dry, but could last longer if there stored correctly in the fridge or freezer. In the fridge I would say about 10-20 years and if frozen they last indefinitely as long as your not taking them in and out..... they like stability and no fluxuations.

Also to note when freezing them, I would not freeze them more than once and only take them out when your ready to grow them but always move them to the fridge to defrost before you move them to an area with a normal room temp.