Meetings, appointments, allday events, and event announcement count as busy time on your schedule. If the calendar doc is flagged "Mark private" it stays busy. With the flag "Mark available" there won't be an entry in the busytime.nsf. Anniversaries and reminders do not count as busy ...

You can set a preference to set the default duration (in minutes) for new appointment entries and meeting invitations. To set this preference, click File Preferences, and then click Calendar and To Do. On the Entries tab, enter a number (in minutes) for Duration of meetings and appointments. The ...