You don't have to answer very quickly, if you come across a question where you get stumped, just say: "That's a good question, let me think about that", and this should buy you some time. Some schools offer mock interviews from people who are professional interviewers. They are brutally honest and very, very helpful, I highly recommend it if your school offers it. To answer your question, I wouldn't go beyond a minute which is a long time when your in front of an interviewer. Remember, they had to go through the same process. They understand the person being interviewed is nervous, just be yourself; that was the advice I got from many dentists beforehand.