how long will they keep?
I dunno about "long" term. So far I've always rotated thru them during the winter months, soup/chili season. The oldest I've got is just over a year, packed last summer. Got a good deal on peanut butter so they'll be gone soon and I'll seal up some more.
For any longer, I'd go with the "hardtack". You could go ahead and make them now before shtf. I'm probably the only person here who's torn between the idea of having grains/mill and something pre-made. I just can't wrap my head around the idea of having time to "bake" with all the other stuff that'll need doing when things go south. Heck, it's a chore now, much less later. And if it fails now, no big deal. Feed to a critter or toss it. Later a fail, well, you know......
Besides, with hardtack, if you don't get around to eating them, you can build something with them, reinforce a door, stuff 'em in your plate carrier or something.