Is this for a college or high school position? It could go from a couple of hours to most of a day. It really all depends on who's doing the interview and what information you want to find out during your time there.
Are you interviewing with the head coach only? The entire staff? Administrators? How well does the interviewer know about you prior to the interview? What is the nature of the relationships between your personal/professional network contacts and the interviewers? There are many variables that can make up that interview time, but don't try to rush it. Make it a point to meet the entire staff while your there. If you don't know the staff ahead of time, make it a point to know something about them before you get there and definitely know something before you leave the interview. After all, you'll be working with them.
Best of luck in the process!