We are unable to see and unable to track changes to the Duration field in the task updates. Other similar fields, such as Planned Completion Date, can be tracked so this functionality should be easy to add. Please add this field to the Update Feeds so we can track changes.
Our task owners and Project Managers make frequent changes to Duration in our implementation and these changes are invisible on the task record. Once this is feature is available, we can track these changes, along with others, in a Journal Entry report.