Grades are recomputed for an assignment in real time as students submit and update their work. You can view grades by clicking the Gradebook button in the left panel.
However, students do not see their grades for an assignment until you release the grades to them. When you have unreleased grades for one or more assignments, you will see a warning to this effect on your course home page; click the Gradebook button and then click the Release grades button under the assignment when ready for students to see the grades for that assignment. (If you have enabled gradebook syncing with your LMS—see below—then note that grades for a particular will not be sent to the LMS until you release them.)
Once grades have been released to students, students can review their scores by clicking My Scores in the left panel.
If Perusall recomputes students' grades after you release them (i.e., because you release them before the assignment is complete), then you will again be prompted to release them if further work by students causes the grades to change.
Alternatively, you can automatically release grades to students by setting the corresponding setting under Settings > General.