Grass seed can last quite a long time, but the life expectancy of grass seed depends upon a variety of factors. In some cases, really good grass seed can last for 2 or more years.
It’s also important to note that the longer grass seed is stored, the fewer seeds are likely to germinate.
How Much Does The Germination Rate Decrease Over Time?
The germination rate of grass seed decreases very gradually. After a period of a few years, the germination rate will likely become too low for it to be worthwhile to use the seeds.
Seed suppliers state the germination rate drops by 10-25% each year. 
After a period of 10 years, this means that only about 10-20% of the seeds would end up growing. Theoretically, one could create a lush lawn with grass seeds this old. But grass seed is relatively cheap, so I wouldn’t recommend keeping it for more than 2 years.
Sealed Vs. Unsealed Storage Of Grass Seeds:
There are two main ways that grass seeds can be stored.
Grass seed stored in a sealed container will last longer. The air inside the sealed container tends to be dry with a humidity of around 13%. This is the ideal humidity level for the storage of grass seeds.
Grass seeds are stored in a bag won’t last as long. A small amount of air can get into a bag of grass seed which will spoil the seed. It is best to get your grass seed in a sealed container if you intend to store it.
Also, keep your unused seed in a cool, dry, dark place to maintain shelf life.
The Type Of Grass:
The type and quality of seed that you’re storing also makes a difference for how long it will last.
There are some types of grass seed that will germinate for up to five years after you purchase it. An example is perennial rye grass.
However, there are some types of grass seeds that tend to have an exceptionally short shelf life.
Here is a study from Oregon State about seed type and germination chances.
How To Store Grass Seeds:
There are things that you can do to prolong shelf life.
If you are storing seed in bags, it’s important that you don’t store all the bags directly on top of each other. This can trap humidity between the bags, and result in a reduced lifespan of the seed.
You also should store the seed in an area where there always is airflow. This will help to reduce humidity.
It also is important to keep your grass seeds out of sunlight. Most bags and sealed containers for grass seeds are designed to keep out sunlight. However, it’s still best to keep it dark area.
An opened bag of grass seed is likely not to last as long. Therefore, it is recommended to store it in the container in came in.
Does Grass Seed Go Bad When Frozen?
Outdoor grass seed storage over winter is possible. As long as the seeds haven’t begun to germinate, sub-freezing temperatures should not have an effect on the seeds.