I setup mine for 30 minutes increment, I change in the "\application\controllers\Appointments.php"
change value 15 to 30 in this function: _calculate_available_hours
$interval = $availabilities_type === AVAILABILITIES_TYPE_FIXED ? (int)$service_duration : 30;
AND this: _get_multiple_attendants_hours
$interval_value = $service['availabilities_type'] == AVAILABILITIES_TYPE_FIXED ? $service['duration'] : '30';