Tomato seeds can last years if taken care of properly. Other seed can last a long time too. However, you want to test germinate them every once and while to make sure they are still viable. This will save you both money and time. You won't need to buy seed if your old seed are good and you won't sit around waiting for bad seeds to germinate.
You can do these for any older seeds you want to check.
Good Luck with Your Garden,
Gary (The Rusted Garden)
Join My FB Page - :The Rusted Garden FB Videos
Join My FB Group - :The Rusted Garden: All About Vegetable Gardening