You need a report that will not only show the amount of appointments in a user or resource mailbox calendar but the total duration of each Meeting or Appointment. (Availability)
This information is stored in the Free/Busy system folder in Exchange 2000/2003 and directly in the user's mailbox in Exchange 2007.
Currently MessageStats does not gather this information from the Exchange Free/Busy System Folder in Exchange 2000/2003 or from the Mailbox for Exchange 2007.
Enhancement request TFS0054345 has been submitted to Development for consideration in a future release of Messagestats