On your schedule you can adjust the minimum increments. The greatest common divisor of appointment length for your clinic is the value you need to use. For example if you have one practitioner who schedules on 15 minute intervals but another who uses 20 minute intervals, you should use 5 minutes as your Schedule Grid value.
In order to adjust this value, head over to Settings then select Schedule Settings.
Then simply select the value you want for your clinic and click Save.