How long can beer grain be stored and still be good for brewing?

It depends, but the short answer is Not very long. Has the grain been rolled and cracked/crushed? Has it been stored in an air-tight bag or container? Have the mice found it yet?! The best bet is to use it very soon after you purchase it -- a few weeks, certainly not more than a few months, under the best conditions. Cracked grain, dry and stored well, several months. Uncracked and dry... years.