Here I am demonstrating a little workaround about how to re-set the Quick Appointment duration from 30 mins (default - selected) to a 60 mins (default), or to your preferred choice, from the quick appointment duration selection box for creating a Quick Appointment.
Go to : Administration -> Customization -> Communication -> Screens -> Open Communication Task filter -> and add the following script in the custom Content Area of this screen.
var y = document.getElementById('SELECTAppointmentDuration') .options.length;
var x = document.getElementById('SELECTAppointmentDuration') ;
for (var i=0;i<y;i++)
if (x.options[i].value == '60') // here you can customise for your option.
x.options[i].selected = true;
Save the screen and once you go back to the calendar tab, the Quick appointment duration will be changed from 30 mins to 60 mins (default selected).
Hope this scripts helps you!