You'd need to iterate all the items in the folder, as in this Outlook VBA or custom form VBScript sample, which displays the total to the user in a message box:
Sub SumTasks()
Dim ns 'As Outlook.NameSpace
Dim fld 'As Outlook.MAPIFolder
Dim tasks 'As Outlook.Items
Dim task 'As Outlook.taskItem
Dim taskTIme 'As Long
Set ns = Application.Session
Set fld = ns.GetDefaultFolder(olFolderTasks)
Set tasks = fld.Items
On Error Resume Next
For Each task In tasks
taskTIme = taskTIme + task.ActualWork ' in minutes
Next
MsgBox "You have spent " & taskTIme & " minutes on tasks.", , "Task Minutes"
Set task = Nothing
Set tasks = Nothing
Set fld = Nothing
Set ns = Nothing
End Sub
--
Sue Mosher, Outlook MVP
Author of Configuring Microsoft Outlook 2003
and Microsoft Outlook Programming - Jumpstart for
Administrators, Power Users, and Developers