It depends. If it's in the freezer in a vacuum sealed bag, it will last a long long time. If it's in a paper bag in the 75 degree living room, it'll last a far less amount of time.
It's sort of like bread. It doesn't really "go bad" all that fast, but it might stale. Taste the grain. If it tastes fresh, it's good. If it has a stale bread taste, they're no good.
I've kept crushed grain, vacuum sealed, in a cool spot for 3 months with no loss of quality.