Confluence should show tentative completion time or some banner pointing to wait for some time if operation attempted under UI is long running. Typical examples around this would be around move page operation which can take a bit longer if the page hierarchy is quite big.
A configurable threshold can be provided which can be tweaked by admin to depict what is a long running task under their Confluence deployment.

Eg - If running page move operation is taking more than 5 secs, then post 5 secs - A banner is displayed to end user that this may take a bit longer and so, please check this page again in few mins.