I buy my grain from Morebeer online. I usually buy a bunch of 10lb sacks at one time. They arrive in plastic bags that I store off the ground on wire mesh shelving in my pantry. I have my own mill so I always buy whole un-crushed grain. The longest I've kept a bag before using it has been 9 months. Usually they don't last that long. In the one case that I did have a bag last that long I noticed no issues at all in the taste of the beer, the mashing efficiency, or anything else. Of course I keep them dry and away from liquids and moisture, but in general they are simply sitting on the shelf. In the bag they are shipped it, but still...on the shelf.
Many times these bags seem to exhibit a minor vacuum effect. They usually keep this effect for at least 6 months or more. After that, the bad may seem to slightly puff up. It has never puffed up enough to give the impression of there being pressure inside. If it did, I would be suspect there is something wrong. That has never happened. I've been doing it this way for untold years without a problem.
If you are talking about storing the grain for more than a year, then I can't comment. But I am confident that any storage in a marginally air-tight environment will be more than adequate.