You can always take enough classes (or hard enough classes) to take up any given amount of time. Or if you're into extracurricular activities, same thing. Consider the quote in bold at the top of this Harvard prof's personal page: http://www.people.fas.harvard.edu/~hgeorgi/

Even if a student takes a more moderate approach, workloads can be highly unpredictable (even to the professor), which creates the same problem.