At this stage, JustMock does not allow you to mock the Stopwatch class as it is being used by most of the test runners. However, we plan to release feature that will make this possible with the next JustMock version. You could expect this by the end of May 2013.
I hope this information helps.
the Telerik team
Share what you think about JustTrace & JustMock with us, so we can become even better! You can use the built-in feedback tool inside JustTrace, our forums, or our JustTrace or JustMock portals.