if you protect them from the elements like high and low temps, and keep them in the dark places say 5-7 years (hometown seeds plated seeds to verify this that were 10 years old and had been stored correctly and had no problem, though i can't find the article now) if you have power you can freeze them and they will store fore a very long time.
me personally i would say you can get 5 years out of your seed stock if kept cool, dark and dry
Generally for each season that the seeds are stored, the fewer of those seeds that will germinate, some plant species more than others.
As Grunt mentioned the viability of the seeds can be extended by storing them correctly, but the best route is to simply collect new seeds from every harvest & rotate the old ones by planting them every season or every other season for the best results.
You have to remember that seeds are actually "living" things (dormant, but still alive) and they will eventually die if not planted, regardless of how they are stored.