Vegetable seeds and flower seeds can be kept for up to a year without any loss in their ability to germinate. You can even extend that to ten years under the most proper conditions.
The moisture of the seeds and the containers that are stored in play an important role in whether the seeds will be storable for long time or not. If seeds are drier, they will store longer.
To ensure that your seeds have good quality, plant fresh seeds from a quality company. If you have seeds left over, and store them correctly, you can use them the next planting season.
In areas like Colorado, Montana and some Midwestern states, most vegetable seeds and flower seeds can be stored at room temperature for a year without losing their ability to germinate. If you bought so many seeds that you have seed for even two or three years, take steps to store them properly, so you can use them when the time comes.
To dry seeds, you can subject them to 100 degree heat for about six hours. You can use direct sunlight to do this. This will bring the moisture content down to eight percent. But you may want to dry seeds in partial shade, if the sun is too harsh directly. Additionally, you could cut down their time in the sun, just a bit.
Do not use a microwave oven to dry seeds. You can use a regular oven, as long as the seeds are not kept above one hundred degrees and as long as you keep the door open. You will want to place your vegetable seeds in containers that are moisture-proof. They must be leakproof, and be able to be put underwater without the seeds getting wet. Use sealed jars or cans, rather than simple plastic bags.
It's interesting that the conditions needed to successfully store seeds are the opposite of those used for germination. Proper germination occurs when oxygen and water are both present, at a good temperature. Proper seed storage results when you keep the seeds at less than eight percent moisture, and the temperature is kept at forty degrees or less.
You can produce "hard seed" accidentally, if you do not reduce the water content to below eight percent. Hard seed will not absorb enough water to have a very high germination rate. When you plant good vegetable seeds, they typically absorb water, and then they germinate and produce plants.
If you make sure not to over-dry seeds when you store them, they should be able to germinate properly when you plant them. If they have been overdried, you will find it beneficial to lay then in a humid area before you plant them.