i was just joking about doing it for three hours. if i have classes that are 2 hours or longer I just treat each hour as a new lesson. The 2 or 3 lessons can be linked but the material needs to be new. At least this has been my experience. Covering one grammar point or topic for more than an hour and a half is too long.
As for length of activities, I try to keep them at 30 minutes or less.