I have heard up to a year if properly stored, I was worried about thay myself, however I blow through 50# sacks of basemalt like a howitzer in a Normandy hedgerows war when I'm upping the pipeline. Never had the opportunity to test the long term storage, all of my non base malts are bought in 2, 5, or 10lb increments and get used up fairly quickly as well.
it's the non basemalts that worry me. I am shredding the 55 pounders as well. I suppose I could look through my brew book and check which specialty malts I regularly use