I think it depends on the university and the subject. The best way to know is to take graduate level classes during your last undergraduate year, so that you know what you are getting into. It also heavily depends on your background, graduate school may even be a step down in difficulty if you switch to an easier subject, e.g. economics or applied statistics after pure mathematics. Each class will demand more work, the assignments will generally be longer, but you also generally take less classes each semester of graduate study.