I have a requirement to monitor how much time is being spent working on cases.

I'd like to see stopwatch functionality on the main Case form that allows users to click start when they work on a case (whether that me typing an email or replicating a problem, conference calls with customers etc) and a stop button when they have finished. The timer should resume from when it was last stopped so we can see a running total amount of time spent.

The timer doesn't need to have a realtime display. Simply logging the time/date when the start and stop button is pressed should be all it needs to be able to get a running total from a report.