There was an issue with Action tracking that the Duration could not be changed. See these old posts:
It was logged as CRID: 34499, Sev 1 due to loss of functionality.
I have tested this in 22.214.171.1247 & 126.96.36.1990 and the issue is still there. Can someone please comment as to when this will be fixed.
There are a couple of high priority defects raised on this issue, CRID 37974, 37970. I do not have a target release for these issues yet.
I suggest you contact your local helpdesk so you can be notified with the "closed loop process" of any status changes of these defects.