I have about 200 lbs on hand. I keep it in ziplock bags to keep dry and bugs away, and in plastic containers to keep rodents out. Keep in basement (60 - 70 F year round).
Most is uncrushed, and I have no problem with keeping for a couple of years.
I do have a few lbs of crushed grain, which is in original sealed bags. I have no problem with keeping it for 6 months or so, but once the bag is opened, I try and use it all at once. If I were to keep it, I think I'd try and use it up in a couple of months.
What goes off? First to go are artificial (or process created) flavors such as smoke. Diastatic power reduces over time too, so it you are mashing and using something like Munich as a base malt, you might need to add some 2-row to ensure you have enough enzymes to do the job. I think malt flavors are lost at the slowest rate of all the aspects of grain.