I can't say how long for sure, but a while back, I let a brew sit in a keg (refrigerated) over a year without touching it. It was a stout and I didn't really like it when I first kegged it. After a year, I needed the keg and it was either dump it or drink it. I tried it and must say that it was really good! I finished off the keg w/ no problems.
The best way to store malt is to combine it with water, hops and yeast... -Charlie Papazian