The View System Activity item is the place to go if you ever want to see what the monitoring service is currently working on. You can choose to show or hide the following activity types:
When you view the running system, you'll notice that running monitors have a start time, but no duration since it hasn't finished yet.
The activity log is purely for your information and can be cleared at any time. When it grows to a length of 5000 items it begins to automatically remove the oldest items.