Gardening Tips (specific for the arid south-west) by John Begeman from the College of Agriculture and Life Sciences at the University of Arizona was the most reliable resource that I could find.
"According to Agriculture Science Experts at Penn State University the (seed) shelf life of some popular vegetables are as follows:
Five Years: Cucumber, endive and muskmelon.
Four Years: Cabbage, cauliflower, eggplant, pumpkin, radish and squash.
Three Years: Beans, celery, carrot, lettuce, pea, spinach and tomato
Two Years: Beets, and pepper seeds.
One Year: Sweet corn, onion, parsley and parsnips."
These times are based on optimal storage conditions which include keeping seeds cool and dry."
These facts are consistent with my own experience. I've used four year-old pumpkin seeds with success. But like I said in an earlier post, my two or three year-old onion seeds didn't yield a single plant.
I did buy a #10 can of already-packed seeds to add to my long term storage. Sellers of these #10 cans suggest a possible 90% sprout rate after 8 years and 50% after 15 years. Those seeds *might* be viable if I have to use them. They aren't all seeds that I regularly plant (like swiss chard), so it would be a gamble as to whether or not I could get all of them to grow. I'm also not sure that my family would eat a veggie that they aren't used to. I think my regular garden seeds are a better bet. I buy a new set of seeds each year, but actually only use the seeds that I bought last year. If you like this idea and are just starting a garden, just make sure to buy two sets of seeds your first year.
This links to a personal experience that Angela shared about her 9 year-old seeds stored in a #10 can: