Hello,
The only way to monitor this would be to check the server activity logs for the time the user used the dial code, however this isn't the most effect use of an administrators time. If you have the office hours set up and breaks for away set up there would be no real need to monitor presence. At the moment the CCE has the ability to show the time the user logged in/out of the Queue but this is about it. If you have an idea for a feature regarding this type of feature in 3cx then I would recommend posting http://3cx.ideascale.com/ here.
Thanks,
Nate