[gogreen437]

I ordered a 50 pound sack of Maris Otter back in February, so about 7 months ago.  I portioned it out into 1 gallon zip lock bags after using it the first time.  I brewed a pumpkin spice ale this Saturday and used the last 8 pounds of the Maris Otter, along with 3 additional pounds I picked up from my local homebrew store to round out my base grain.  The stuff I got from the store definitely had more of a crunch to it.  The old Maris Otter still had a little crunch, but was a little mealy, though it didn't taste bad by any means.  How do you know when malt has gone bad, and how much does your beer suffer when using old grain?

[bo]

Taste it. It should be crunchy and not taste stale. If you aren't sure, taste some known good grain and compare. 

[tomsawyer]

I do think it can lose a little aroma over time without becoming officially stale.  Not sure this translates to less aroma in the beer but I do try to rotate stocks and keep sacks tightly wrapped and in rubbermaid containers.

[bo]

If it's kept in a dry environment and uncrushed, it will  stay fresh for a long time. I have used some 2+ years old and could tell no difference.