When you create a WO directly, or update the primary indent type on the WO it takes the duration correctly from the duration of the incident type whether there are tasks or not on the incident type. When you create the WO by converting a Case to a WO it takes it from the tasks attached to the Incident type. If there are no tasks with a duration, the WO Primary incident Estimated duration is then zero even if teh incident type itself has a duration. This needs to be consistent and a WO created from a case should also have the duration set to the incident type duration even if there are no tasks on the incident. This would be the correct behaviour as organisations often do not need tasks on incident types.