Well, there's lots of different opinions on that subject. My experience is based on the 50 lb bag of crushed pale malt I bought a few months ago. I knew it would be a while before I could use it all up, so I put the extra in large, thick plastic bags, twist tied them, and put them in the deep freeze. When I pull a bag or two out to brew, the grains seem as fresh as they did when I got the sack of grain back in March.
I think the key is, don't leave them exposed to air, and keep them in the freezer if possible.
That reminds me. I've only got 15 lbs of grain left... I might need to order another 50 lb sack of grain. Nothing keeps you motivated to brew like the smell of fresh malt.