The default duration is dependent on the view you're using and what you
click.  If, for example, you double-click a day in the monthly or weekly
view, your event will be an all-day event.  If, instead, you are viewing the
daily view and double-click, your default event duration will be your time
scale setting (with a minimum of one-half hour).  If you preselect the
interval in the daily view and then click New>Meeting Request, the duration
will be whatever you had preselected.