Q: What's the best way to save seeds from year to year? And how long can I expect to keep them before they won't sprout anymore?
A: The best place to save seed is inside envelopes or packets stored in a screw-on jar in the refrigerator. It also helps to place a packet of silica gel or dry powered milk in the jar to absorb moisture.
Second best is any other cool, dry spot you can come up with.
Some seeds store better than others. You should be able to get at least a second year out of just about any seed you've bought or saved, and some will remain viable for 5 or more years.
Lettuce, corn, parsnips and onions are examples of species on the short-lived side. Cauliflower, cucumbers, eggplant, melons and radishes are more on the long-lived side.
If you're not sure what's viable and what's not before planting, wrap a few seeds (10 is a good, round number) in a damp paper towel and place in a sealed plastic bag at a temperature ideal for germination. That should be listed on the seed packet. If not, around 70 degrees is a good average for most plants.
Check the seeds after about a week and then every day or two afterward to see if many/any are sprouting. Most will germinate in 7 to 14 days. If you've got nothing after three weeks, you can give up on most seeds at that point.