Grass seed doesn’t have a firm expiry date, but like most living things, it can lose some germination power over time. As long as you keep your unused seed in a cool, dry place (indoors, sealed bag / rubbermaid bin), it should stay viable for at least a year — sometimes longer. Avoid humidity, direct sun, and extreme temperatures if you can. A basement is often better than an outdoor garage or shed.
We always recommend using it sooner rather than later for the best results. If you do decide to store it for the following season, consider using a slightly heavier seeding rate when you plant it.