A lot depends on how it's stored. Leaving it in a garage, where it's
exposed to heat and humidity, is not a good place. It will last a lot
longre stored in a cool, dry place. I put mine in a closed garbage
can, with a container of the dessicant moisture absorbing stuff inside
and keep it in the basement.

The germination rate will decline slowly over time. I've stored it for
several years and gotten good results. If you have enough of it,
prior to using it, I would count out 50 seeds, plant them in a
container, and then count how many germinate. Then you can adjust how
much to put down.