The document of Ganesha Sridhara clearly shows how to add a value to a service desk field. But I donâ t think it offers a way to do time tracking.
I think the best way to get an overview of the different elapsed times of every status would be a reporting tool. For example the Business Objects based Westbury Report Manager allows you to calculate the time between two different time stamps from the history lines. This way you could get a clear overview of the elapsed time of each status.