By default, messages will be marked as read when selection changes.  If you want to mark items as read after a specific time,

  1. Click “File” tab and choose “Options”.
  2. Choose “Advanced” and click “Reading Pane” under Outlook panes.
  3. Uncheck “Mark item as read when selection changes”, check “Mark items as read when viewed in the Reading Pane” and then enter a number in the “Wait __ seconds before marking item as read”.
  4. Click “OK” on the dialog boxes to change the setting.