How long do seeds last?

The most important factors in storing seeds are temperature and light exposure.

For seeds it's really important that they are stored in a dark place with a constant or semi-constant temperature (ideally 4-7 Celsius).
Best place to store the seeds for periods over 1 month is in a no-frost refrigerator (with moisture control) at 4 degrees Celsius.
If stored in a no-frost refrigerator the seeds have a lifespam of 3 years, but beyond the three-year mark, their germination rate will begin to decline.

Sometimes seeds can stay viable for over 10 years, but it's a rare instance.