I have stored grain (even milled grain) for a year without noticeable degradation in finished product. Of course I store them in airtight containers (usually homer buckets) in my basement (cool and dry).
I'm always surprised at the people who freak out if they didn't mill their grain seconds before mashing in... my experience has been that it's not a big deal to store grain or even keep milled grain for some time and still have it be good.