For world record, a few geniuses finished them in a year.

Fastest for someone I know personally: three years from one of the top universities in the US. This is realistic. Can't say they got much out of the program, though. All-in-all, it seemed like a waste of time.

I spent a __long__ time in graduate school, taking interesting courses, travelling the world, and just on learning and intellectual exploration. If you want that, get a Ph.D. Just realize your advisor is just that -- an advisor, and not your boss. You don't need to do what they tell you to do. If you want a slip of paper, consider why you want it, and if you wouldn't be better served by an MBA, MS, or something else. A Ph.D is not all that important, career-wise.

Rushing a BS makes complete sense to me. A Ph.D? Not so much.