Is there any way that we can change the default duration of an appointment
from 30 mins to 20 mins.
Set your calendar time scale to ten minutes (right-click an empty area of the
calendar, choose Other Settings, and change the Time Scale drop-down), then
preselect two time intervals before clicking New. Without a preselected
duration, clicking New in a view showing the time scale should give you a
default duration matching the time scale. I tried it with ten minutes. There
is no way to specify 20 minutes as the time scale.