No need to calculate, it will tell you.
Go Online with the controller. Right click on your continuous task (default name is MainTask) and select Properties. Click on the Monitor tab and you will see the Last Scan and Max Scan times.
You can do this for each Task and for each Program though you'll notice some are in Milliseconds and some are in Microseconds.
Generally, the scan time of your Continuous Task is an indication of the overall scan time.
There are other ways to get this information, but that's one.
OG