Stored correctly, Sensi seeds should have a lifespan of at least two years. This is certainly not to say that after two years all your seeds will be dead, but beyond the two-year mark, their germination rate will begin to decline.
Seeds should always be kept dry, cool (close to 4º C if possible) and dark. The refrigerator (NOT the freezer) is a good place to store seeds, preferably inside a film canister, to avoid light and moisture condensation.
In the absence of a fridge, the back of a closet is OK, as long as your seeds are not exposed to moisture, heat or excesses of light.