In order to change the default hours in EditAppointmentDialog I would suggest that you use the AppointmentCreating
event which is fired only when creating new Appointment. You should check the type of the ActiveViewDefinition in order to change it only in MonthViewDefinition.
Please see the code snippet below:
sender, AppointmentCreatingEventArgs e)
var app = e.Appointment
app.End = app.Start.AddHours(30);
Hope this helps.