Keep track of how much time user has spent studying card, deck, folder, and overall. This should include the time spent entering the card, the time doing quizzes, flip cards, etc.
It can't be 100% perfect as someone might accidently leave their window open but it could do things such as ask the user if they are still there if they take more than 30 seconds to answer a question, don't type anything on a card for a certain amount of time, etc.
How do you see a tracker of how much time you have spent on our site proving useful to you and your overall study process? Are you interested in seeing some kind of pop-up appear after periods of inactivity so as to prompt users before logging them out?
Any further explanation you could provide would be appreciated.
I think its useful in that you can tell how much time you have spent studying and gauge whether you think that is a good amount of time. Some instructors and institutions will tell you how much time they feel you should spend studying per credit. The popup suggestion was just a mechanism to try and make the time calculations more accurate as it obviously cannot be perfect. One site that has this implemented is MyFinanceLab which provides the total time by assignment, test, quiz, and overall.