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.
I'm interested in this as well. I'd like to buy bulk but don't feel like dipping into the pocket (again) for a grain mill right now so would like it pre-milled. I figured I'd buy locally (more $) until I got a mill but if it'll last 6+ months I'd be good for bulk. Freezer was my thought as well...
the sooner the better from what i have always been told. i never let mine sit more than a week once they have been milled for brewing. i look at it like anything else. once the process starts, the clock is ticking.