You will need to click on Reporting, then on Report Builder.
Next, select Course Report → Summary of a Course → (Choose a Course) → Summary of This Course → Run Report. You will then see the total amount of time the a learner has spent in the course under column K.
For a more granular overview of a learner's time spent, change the options to Course → Summary of a Course → (Choose a Course) → Summary of a User in This Course → (Choose a User) → Run Report. This will break down the time spent, module by module.
You can get the same report by choosing User Report → Summary of a User → (Choose a User) → Summary of This User in a Course → (Choose a Course) → Run Report.
If you prefer a more visual report, you can click on Reporting, then on Visual Reports. From there, click on Courses.
You can use the drop-down menu to choose which course you would like to run a report on.