I have had seeds stored in my freezer for over 3 years, I pulled out a pack earlier this year and germination was so good I had to give seedlings away. My father always stores his seeds in the deep freeze, anything and everything regardless of what kind. He always has a bountiful garden each and every year.

Seeds can last a long time if they are properly stored. I read about a 2000 year old date palm seed found near the Dead Sea that successfully germinated. I keep mine in freezer bags with the air squished out and placed in a metal container in the refrigerator.

The short answer? Between two weeks and several thousand years, depending on the species and condition they're stored in ;)

Garden seeds just kept in a drawer are usually good for years.

Just looking quickly I would say the only ones apt to keep poorly are the onions, and these will still probably last for a couple of years. I think most of the others will keep for several years at least. Lettuce MIGHT be a problem, I am not sure. Just keep them somewhere dry and cool(room temperature or less).
JMO,
Tom

Thank you for the guidelines and strage tips! I will try to make use of the onion seeds in the near future. Luckily, I don't think I have that many of those compared to some of the other packs that have 1000-2000 seeds.

if the seeds are put in the sealed container and no bugs can damage them it will last for long.

If in doubt whether the seeds are good or not, I usually test the seeds by placing them on a MOIST paper towel, folding it over several times. Place in a small Ziploc plastic bag and put on top of the refrigerator. This method is explained on page 41 of Garden Way's JOY OF GARDENING by Dick Raymond. On July 4th I placed two seeds of GAYLIA MELON 409 which I obtained from Thompson and Morgan on 07-15-1989. That is correct almost 20 years ago.
One of the seeds germinated on 07-10-09 and I planted it in a 2 3/4" plastic pot filled with moistened germinating mix. The seeds were kept at room temperature all these years in the original inner foil packet and outer paper packet. My reason for wanting to replant this variety was that it is the standard seed from which I can obtain seeds to save. Galia melon is now available as a hybrid although some seed providers have the OP available. I tried this method with 10 Melon Ogen seeds with 100% germination. I just checked the invoice, and it is listed as Melon Gaylia F1 Hyb even though Standard Seed is listed on the right hand margin on the back of the packet. Will have to rethink about saving the seed if indeed it was a hybrid.