[jofer]

Depends heavily on how you store them.

Germination rates drop over time. Storing seeds improperly will increase that rate (i.e. even fewer seeds will sprout after a give time). You'll more-or-less always get a few to sprout, but you can't count on getting the vast majority to sprout unless you've stored the seeds properly.

If seeds are stored at high temperatures (room temp or above) in non-airtight containers, they typically have a "half-life" (non exact) of a year or two.

If you store the seeds in an airtight container in the freezer, they'll keep more or less indefinitely. (I'm guessing, but let's say a "half-life" of a decade or so.) There are instances of seeds preserved in permafrost sprouting after tens of thousands of years: http://en.wikipedia.org/wiki/Oldest_viable_seed

All of this depends on the type of seed, as well. Take my numbers above with several grains of salt. They're from general experience growing up on a farm, not any definitive source.