I have thought about this because I have some things in the BOB that have a limited shelf life, like energy bars, crackers and beef jerky. At first I kept them in the BOB, but they eventually reached their shelf life and had to be discarded, and replaced. So what I did at first was take all the items like that and put them in a ziplock and keep them in the freezer until needed. However, that means digging them out of the freezer on a moment's notice, and things tend to get pushed back to the back of the freezer...so that is not a really good solution.
If I pack several ziplocks with those items and rotate them, say weekly, keeping all but one in the freezer, and keeping one in the BOB, then that should extend the shelf life considerably, but not indefinitely. Or, I could just eat the items in the BOB before they reach their shelf life and put a fresh pack from the freezer in periodically. Then buy new supplies when the freezer supplies get low.
As far as the other items, some have been in there so long I don't remember everything off the top of my head that is in the BOB. And I may have raided the BOB for things like Neosporin and Multi-tools. I have a long term BOB that would be a pain in the Butt to repack (too big to carry on foot, so it's for bugging out in the truck). The normal BOB however wouldn't be that difficult to repack. I need to think about a rotation scheme with the non-perishable contents.