Interview times vary significantly from school to school. At Michigan, for instance, the interview process takes nearly two hours because of their multiple-mini interview (MMI) format, which consisted of ten, ten minute interviews.
Other schools, such as my interview at Iowa, have one interview that will last between 20 minutes and 40 minutes.
Some may have multiple interviews, each lasting 20-40 minutes (such as Oklahoma).
Typically, interviews will consist of a very small portion of the day. You will usually be given presentations throughout the day on the curriculum, financial aid, research, extra-curricular activities, etc. Lunch will be somewhere in the middle of all of this.
I have actually enjoyed my interviews thus far. The staff are usually very welcoming, and I have gotten along well with the other people who are interviewing.
Good luck on your interview process!