Grades are recomputed for an assignment on an hourly basis, until the final opportunity for students to change their scores has passed. (If you have neither a post-deadline reply window nor a late annotation period for your course, this is just the assignment deadline; if one or both of these periods are set, then the assignment will not be graded until both have passed.)
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.) 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.