ADHD is different for everyone so results will vary from person to person.
For me, it was a couple days before I had enough of a "base"
I didn't know how much of an effect it was having on me until I stopped for a couple weeks. It was more my wife that noticed...
I have noticed that my hyper fixation on ruminating has disappeared almost entirely and my sleep is better.
I'm not better at staying on task necessarily, but I'm better at getting back on task, starting tasks, prioritizing tasks, etc.