Three years.
I'm still in my head a lot, and I still think a lot, but it has gotten much better. At the moment, I'm not on medication for my ADHD because I'm nursing. I stopped taking it when I was trying to get pregnant. One thing that has gotten better is that I don't get as upset about the things I don't do well, or about my hyperactive mind. I let myself off the hook, instead. For example, I don't even try to finish a book I don't get completely hooked into, because it's just too hard, and I have other priorities than creating accommodations for something I don't really need to do. (My child is a much bigger priority, for example. Feeling well is, too.) I am trying to get my head around how I will get my papers cleaned up and filed or thrown out, but I try to step back from it when I get myself worked up about it. If I need to get a check deposited, that actually needs to get done, so let's put our efforts there, collecting deposit slip and envelope and actually getting it to the bank.
It may be that all the mommy-related sleep deprivation keeps me from thinking as much, but I'm not sure and I don't recommend it as an ADHD management strategy. I do know that I know myself a lot better, and that was the beginning of letting myself off the hook. When I was first evaluated for ADHD, the psychologist who diagnosed me said, "now give yourself three years. It takes at least three years to get to know yourself now."
I think that was one of the wisest, most useful pieces of advice I've been given along the way. Knowing about my ADHD has been such an key to understanding my experiences and myself, and integrating that takes quite a while. Every situation, every context, takes on new information and new perspective. The past is rewritten slowly with this information as well, but again, it takes time. In some sense, the period after diagnosis seems a lot like the exploration that happens for most people around the time they might be in college; getting to know your likes, dislikes, limitations, boundaries, what you'll put up with, what you are good at doing, what and who drive you nuts, that sort of thing. These are things that finally started to become clear to me after my diagnosis, and with some medication on board, which helped me to be more aware of what I was experiencing. The diagnosis gave me a whole lot of information, and the medication slowed my thoughts down enough that I could even know what they were. That was the beginning.