Started at noon, ended at 4:40 (interview was scheduled at 3, but started late) - make sure you add traffic into your timeline. Coming in and leaving on the same day seems nuts by the way, I would come in the day before, stay the night and leave on the day of the interview.
My interview day started at 10:45 and ended an hour early at 3pm. There's UCLA adcom actually sets two group of interviewee. One group will start early at 7:30 and end early, and one group will start later at 10:45 and end at 4pm. Good luck!