Are the B12 injections meant to be ongoing, or only for a few months? How will we know when it's time to stop?

Ongoing. I still continue them to this day. I often take days off, and I take weeks off every now and then, and my health doesn't decline when I do. So technically I don't "need" them anymore and I can go without them. But I personally don't plan on ever stopping them entirely because of how much they have helped me. Dementia also runs in my family, and I want to do everything I can to avoid that in the future. But theoretically, they can be stopped once your OAT markers fall within the optimal range.