When I add Duration to a view in the Activity entity, why is the data that I expect not displayed in the view?

There are two attributes used to track duration: Duration (scheduleddurationminutes) and Actual Duration (actualdurationminutes). Your Microsoft Dynamics CRM Online users might be entering information for one attribute, while the system is set to display the other. Data for each activity is stored in a generic activity entity and specific entities for each activity type:

Activity entity
Stores generic activity information such as the owner, scheduled duration, and actual duration of the activity. There is no form for the Activity entity; data is entered on the specific activity forms. There are views that combine all activities into one view.
E-mail, Letter, Fax, Task, Phone Call, Appointment, Campaign Activity, Campaign Response, and Service Activity entities
Store specific attributes required for the type of activity, such as the From and Cc fields on an e-mail message or the Location of an appointment.

Depending on the activity type, different duration information is collected:

E-mail: Actual Duration (actualdurationminutes)
Letter: Actual Duration (actualdurationminutes)
Fax: Actual Duration (actualdurationminutes)
Task: Actual Duration (actualdurationminutes)
Phone Call: Actual Duration (actualdurationminutes)
Appointment: Duration (scheduleddurationminutes)
Campaign Activity: Actual Start, Actual End
Campaign Response: no duration attributes on form
Service Activity: Duration (scheduleddurationminutes)

If you add Actual Duration to an Activity view, no data is displayed for appointments, campaign activities, campaign responses, or service activities.

If you add Duration to an Activity view, no is data displayed for e-mail, letter, fax, task, phone call, or campaign response activities.