A rough scale might be:

Mild ADHD: You're more distractable and forgetful than most. You have techniques to get around this such as reminders, planners, assistants etc.

Moderate ADHD: You struggle with important task completion all the time. You've experienced consequences from it such as job loss or dropping out of college You may have depression or anxiety related to your ADHD

Severe ADHD: You're in prison because of a violent outburst caused by your emotional disregulation. You're homeless because you can't hold down a job or a relationship. You're dead because you crashed your car while distracted.