Click the Menu Icon (≡) and then click Setup > Setup Appointments & Emails
Enter the number of minutes into the Booking Interval field
Intervals can be as low as 5 minutes
Pro Tip: The Booking Interval is for the entire Company and these intervals will appear on the Calendar. If the Booking Interval is 15 minutes but a worker would like to take appointments every 30 or 60 minutes, that is a preference located in Setup > Setup Workers > Worker Details > Services tab and select the value under Book Appointments Every # of Minutes.