You should reserve at least a 4-hour time period for the defense. Most defenses take a minimum of three hours and can take up to four or more. You want to be sure to allow ample time for set-up/clean-up and questions from your committee.
Plan for 10-15 minutes for set up (refreshments, seating, media), and 10-15 minutes for clean up. It is your responsibility to make sure the room is clean after the defense.
If possible, avoid scheduling the defense back-to-back with another student’s defense.