Your teacher must mark that you attended in order for the time you spent in a live instruction class to appear in your Study Time. It can take up to 24 hours after your class is complete for this to update.
Anything else you do in Digi English that is not specified above, like simply viewing a page or logging into Digi English, does not get included in your Study Time.
Even if you were doing something that is specified above, like browsing the Grammar Guide, once you go idle for too long, we stop the counter. But we always start the counter back up once you are actively working in those areas again.