Although length of an interview is suggestive of a "good" interview, we have to remember that each interviewee is different and the interviewer is looking for answers to specific categories. For Northwestern they're trying to gauge your maturity, sincerity, listening skill, career progression, project management, intellect, career focus, and etc.
So even though an interview may be long for others, it doesn't necessarily mean that the interview went well.
Some examples that would cause interviews to be longer:
-Interviewee is rambling, not concise with answers, and has long moments of silence.
-Interviewee's answer does not give the adcomm insight into the traits listed above.
-Interviewee asked more question than interviewer.