A agree with Generator. Every tut is different. Some days everyone may understand everything completely and the time the teacher (possibly) left for questions will not be needed, in which case you may get let out early, or the teacher may discuss what you will talk about next tut.
Or the tut may be quite hard and there will be a lot of questions, in which case it will go as long as it possibly can, questions will probibly end up being asked outside because the room will probibly be booked for another class after.
So, basically, there is no way to tell whether a tut will end early or venr un slightly over time, unless your teacher actually tells you that you will be let out early.