The new Case Handling Time Widget is currently limited to the out-of-the-box Customer Service Workspace Model Driven App. Microsoft have confirmed that there are currently no plans in place to make this available to custom Multi-session Model Driven Apps. This should be universally available for use within any custom Case based multi-session app. I am not able to use it because we don't want to customise the out-of-the-box version of Customer Service Workspace and I cannot add it to my own version of that App.