I had some milled Paul's chocolate that was a bit over 3 years old when I finished it off. It was still fine, but not being a base malt makes a huge difference, and the extremely dry climate here makes an even bigger difference. I haven't notice any degradation of unmilled stuff until well over 2 or 3 years. When I got back into brewing a few years ago I still had some umilled pale, crystal and chocolate malt left from my previous foray in the mid eighties, the pale was okay but less than ideal, the crystal and choc, were fine.