I agree with those who say this is a very silly question. Those with a strong work ethic who spend a lot of time working are the ones who work the most. They will carry this through regardless of the stage of their career.
About the only thing that will be weeded out along the way are those who goof around and play all the time and rarely put any effort into their work. They will wash out before graduating. But, I have to assume you're referring to those who are successful enough to progress to the next level.
What changes along the way is the TYPE of work one does. Undergrad...studying, doing homework, attending classes, a minimum-wage type job on the side. Graduate student...studying, homework, attending classes, lab research, dissertation writing, with the time distribution shifting from coursework to lab work as they progress. Post-doc...lab research, supervising undergraduate and graduate students in the lab, writing papers, learning to write grant proposals, maybe teaching a lecture or two in their subject area to gain teaching experience. Assistant professor...lab research, supervising technicians, undergrad and grad students and post-docs in the lab, helping their students write papers, writing grant proposals, teaching a block of lectures in a course or maybe responsibility for one entire course, some time spent on committees. Associate professor...about the same as assistant professor, but with an established track record to make grant writing easier. Full professor...same as assistant and associate professor, except may hire an assistant professor into the lab to supervise more of the every day work happening in the lab while getting established, and in place of being in the lab and directly supervising lab personnel as much, time is shifted toward university administrative responsibilities such as chairing committees, chairing a department, serving as director of a program, etc.
As Astronuc pointed out, yes, it's generally assumed that each class hour includes 3 hours of work. That applies to both students and professors (at all levels). For the students, it's 3 hours of studying, doing assignments, reading, etc. For professors, that's 3 hours to prep lectures, write exams, grade exams, enter grades into gradebooks and report them to administration, and meet with students outside scheduled class time.
And, yes, just as some students will put in more than 3 hours studying for every class hour they meet, some professors will put in more than 3 hours prepping for classes or meeting with students outside class time.
Welcome to being a grown-up. You get to stop working when you retire or die, whichever comes first. Even then, we get some Emeritus Professors who still won't stop working well past retirement age.
