The classic answer for all things PhD is that "it depends". Freshman-sophomore-level introductory classes tend to take more time, because students still don't know how to study. Office hours tend to get real busy right before exams. read more
Ideally, a PhD student should work 0 hours a day. If you do the work you love, you won't work a day in your life. Of course, most people have to overcome tough obstacles in the beginning of their scientific careers, but it is what you make out of it. read more