Average MBA interview = 30 minutes.
I would not say that you can judge an interview solely on the length of it. Sometimes I think of an interview a lot like the GMAT. You can easily feel like you are doing GREAT and still not get the job, or you can feel like you BOMBED it and you get the job. Really, it is hard to know. I wouldn't read too much into the duration because half the interview should be about you, and the questions you had for the interviewer. They are not only interviewing you, but that want to know that you have questions for them.
It's way to easy to spend time dwelling on "how the interview went" when there is nothing that can be done about it now. Move onto the next step, and continue doing whatever you need to do to get your dream admit.
Good luck!