I use the pomodoro technique. My plan per day is to study in 20 pomodoro sessions of 25 minutes each, with their proper breaks between them.
I've been trying to pass such exams since 2018, but I got diagnosed last year, so until then I didn't know why I couldn't do anything. Then I didn't have any tools. Now that I have the tools, those are fault.
As reference, my brother, who doesn't have ADHD, is able to study for 12 hours in his peaks.