There have been some discussions (I think in the
food preservation forum) about long-term storage of beans. I recall that the main issue is they get increasingly hard and it takes longer and longer soaking/cooking time to make them edible. I don't know that there is anyway to tell if a particular been would get too hard for any particular mill unless you already have the aged beans on hand. I would speculate that humidity and other storage conditions would affect how had they get over time also.
Maybe better to plan on having fresher beans around?