When I go to a Student's Progress --> Content --> Topics Visited I believe the time I see only reflects the last time they were there.
For instance: I can see a student has visited a Grammar lesson 8X but has only spent 1m 44s there. This does not make sense since she would have watched a video, completed the work and submitted it.
I believe it would be far more helpful if instructors could see the cumulative time a student spent on Topics.