Cannabis seeds are quite hardy and will last for at least 2-3 years without any drop off in viability, if they are stored correctly.
The most important thing is to keep them dry. Ideally in a tightly sealed container like a camera film tube or a clip lock Tupperware container.
Secondly they should be kept away from sunlight.
Lastly, they need to be cool, preferably between 4-7 degrees Celsius.
Ideally if you put them in a small sealed container and leave them in the door of your refrigerator that should see you through for 1-2 years without any problems