"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."