Last time I used corn (popcorn to be exact) I soaked for about 24 hours replacing the water several times over that period with fresh hot water. Then I simmered for 30 minutes or so and drained, turning the corn every minute or so allowing the steam to evaporate off. When you load it into the jars, you want a bit of moisture, but not so much that the kernels stick to the jar or each other. Quart jars I ran for 90 minutes... wouldn't go bigger than quart size as it'll take longer to colonise and more chance of contams.
Honestly though, if you've got a pressure cooker, start making your way to WBS (wild bird seed) like millet. Much more surface area which will in turn lead to much faster colonisation when you shake your jars at 25%, and again when you spawn to HPOO or whatever your substrate will be.
Popcorn can handle a more vigorous boil without busting the kernels than WBS can, so if you're still going that route you can be a little less cautious. If you start to see a few break open, stop the cooking process and proceed to draining/jarring.