If you are thinking CPU Time in sense of what you can see in Task Manager, it is total time scheduler gave to process.
Each thread in system gets some time to run. If there is no work to do, it will return immediately and thus it will not use that time. In case there is work to do, it will run until scheduler stops it and gives control to another one. That slices of time that thread was actually running are cumulated together and this is CPU time. As thread does more work, more time slices it spends and thus CPU time is higher.
CPU time is not defined in CPU cycles but in "natural" time units (hours, minutes, seconds...).