Any ideas? Apologies if this has already been covered, but using different terminology. Sometimes you don't know what you're looking for until you find it
You are not the only one asking this question. I think you might do some searching of the forums, but in particular look at activity tracking. There are a lot of difficulties trying to figure out how much time people spent on a course. The most ingenious solution I heard about was letting the screen dim after a few minutes of inactivity. If the user clicked, the screen brightened and it was logged as continued activity. If there was no response by the user, then it was logged as "user inactive". But this kind of solution would not be available in the basic moodle install.
You could try to do some statistical analysis of log data, but that can be difficult.
It is much easier to track completions or transactions. You can use activity logs to show you how many times a resource was opened or when an assignment was turned in. You can show how much time a student took to complete a quiz.
If you do find an answer, the rest of us will want to hear about it!