I had an interview where the interviewer asked me 3 questions, asked where else I had applied and ended with, "I'm sure you'll be accepted here". I asked two questions and shook his hand, all for a grand total of less than 10 minutes and sure enough, I got in. I don't know if there is any data on how interview time correlates with acceptance rate, but in my very limited experience, it seems to make no difference. Putting myself in an interviewer's shoes, if an applicant is interviewing well, I'd see no reason to prolong the session longer than necessary, but if somebody is giving a poor interview, I may try to ask them a few more questions to see if they can salvage it.