I've put grains in a zip-lock bag and then into an "air-tight" tub and it's stayed good for months. Now how air-tight the tub is I can't say, but I was worried and it turned out there was no reason to be. With that said, I would say the sooner you can use them the better.
I purchase grain directly from the wholesaler. The wholesaler informed me as long as I store my grain in a dry environment. The grain will keep indefinitely. No matter whether the grain is milled or not. I buy 50# milled bags of base malt 3 times a year. I only use 8lbs of malt for each 5 gals of ale. Our computer room has the lowest humidity.