It depends largely on how you store your grain. I've used 3 month old milled grain without a problem. Don't believe anyone that tells you you have to use it in a few days, it can keep for weeks if you store it cool, dry and away from air.
I've used cracked grain that was 6+ months old and it still made great beer. However, I did keep it in it's original sealed packages from BMW and in my kegerator at around 36 degrees.
I've used cracked grain that was 6+ months old and it still made great beer. However, I did keep it in it's original sealed packages from BMW and in my kegerator at around 36 degrees.