I think it depends. Sometimes a short interview is okay- they don't have a lot of questions for you. How short was it? 15 minutes, 20 minutes, 30 minutes? A long interview doesn't mean anything- I recently had a 3 hour interview that went really well- I clicked a lot with the hiring manager. I was very confident that I would get the job, but she chose someone with more experience. A long interview is usually a good sign but not always. I would go by your impression of the interview. If it seemed to go well, I would be optimistic. But, if it seemed to go badly (they were disinterested or seemed distracted) then I would be less optimistic.