The table below outlines a rough guideline to how long seeds will live or survive once the seed packet is opened and also in many cases from when the seed packet arrives (un-opened)
Seed longevity depends on a number of factors the greatest one of all is how fresh and how well stored the seed was from the seed supplier at time of purchase (not all seed suppliers care for their stock as Growseed does)
Some gardener’s find it impossible to use all the seeds sold in many seed packets such as 800 purple sprouting seeds however given that those seeds will germinate fine for a number of years (5 in fact) it’s a worth while investment from the start.
How to store seeds once opened
Fold the packet over or re-seal the packet where possible (again this depends on manufacture) place the packet in an air tight container and away from extreme temperature changes (radiator or a metal garden shed for example) ideally you want to place that container in a cool dark location. Bottom fridge door is ideal, seeds will live for an almost indefinite time frame if stored at a constant 2 Celsius. This is not something a normal house hold fridge can do however it will help keep the seeds alive for longer!