Palmer's book says six months, IIRC.
I had some which I bought in 6/2009, and it was noticeably darker than the same LME purchased 1/2010.
I'm not sure I agree in re: the grain, however. I have quite a bit of grain that's way past six months, but it seems fine to me. It's stored in sealed plastic containers. I did a french-press mash of some two-row that was probably 18 months old (crushed when purchased), and I compared the tea from that to some tea made from fresh-crushed, just-purchased two-row, and they tasted the same. In fact the old grain probably tasted a little better.
Yes, not a scientific test in any way, but for what it's worth, I've been using that old grain for batches, and the beer is good.