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