You do a lot more than interview. The first two hours or so will be a presentation about the school, basically a time for them to brag and tell you how the school operates and whatever they feel like. Then you will go on tours of the school, usually led by students. Around 12:00 you will have lunch with faculty and students. Then you will have your interview scheduled in 30 minute blocks. However, you will have 30 minute blocks between your interviews to allow you time to get to the next one, and allow interviewers to fit you in their schedule. So for example, from 1:00 - 3:00 might be interviews. So you could have yours scheduled for 1:00-1:30 & 2:00-2:30, or 1:30-2:00 & 2:30-3:00. Then usually there is another short get together to have a lecture about w/e the school feels like or hang out period in a lobby with drinks/cookies.
Thus, most interviews take the whole day.
