I know tracking time spent is a highly requested feature. According to this post, it is something they are working on. (Time Spent on Platform)
For now, you can track the last time they made progress when you are looking at their progress (in the progress tab) by mousing over the name of each student. It would be nice if you could see it for all students at once rather than having to mouse over each name, but maybe that’s part of what they are working on for future updates.
Hope the progress tracker helps you a little bit at least.
Mike