Hi Martina. This is a deceptively difficult question to answer. It really depends on how you define the problem and what assumptions you make along the way. It is unlikely that you will get a very accurate number, and any values you derive will be up to interpretation.
In theory, you can get the numbers from running a customSQL report that analyses the logs and looks for teacher triggered events that accord with assignment grading workflows.
............. but there is a big elephant in the corner of this room
Let's assume that it is an assignment that requires a student to complete and submit a Word document attachment.
In the logs, you see that the teacher has downloaded the attachment. 2 hours later, the teacher goes back in and adds a grade for the student.
Do you count these two hours, or not? Did the teacher spend some or all of this time reading and grading this submission, or have they gone off and made dinner? You have no way of knowing.
What happens when a teacher marks assignments in bulk. I know that some teachers will wait until all students have submitted, then download all assignments and mark them offline, so that they can compare one student to another. It might be a week until they come back in and record a mark for each student ...... how do you count this time?
....... here be dragons.