I'm curious where does the line cross from having a mental disorder, to insanity?

Reality: I believe that one would stray into the insanity territory when they lose touch with reality.

Great question: You will get many different answers to this question. I like the answer about losing touch with reality (e.g., having delusions, paranoia, hallucinations). Also consider possibilities such as can't tell right from wrong, having irresistible impulses, experiencing extreme jealousy, inability to learn from experiences, inability to have any empathy, enjoyment of hurting others . .

It doesn't.: "Insanity" doesn't exist in medicine. "Insanity" is a legal term. In medicine, the nearest term is "psychosis." The term "mental disorder" is so vague as to be meaningless. It's as useful as telling your mechanic you have "car trouble." It can mean almost anything. To get a "real" answer, you need to ask a "real" question. Any answer you get to your current question will be nearly worthless.