Life as a master student vs PhD student
There's no single "correct" answer in the life of a graduate student—the culture varies from university to university, department to department, and even group to group. It's also a function of the PhD student as well.

One thing to note is that, as a CS student, you're working in a field where it is very easy for students to do their work in environments outside of the laboratory. (Compare that to, say, experimental biology or chemistry, and the freedom in work environment is pretty stark.) Consequently, you don't really have a good feel for how many hours the students are working in aggregate—they may be spending a lot of time working from home, or in the evenings or on weekends.

At the other end of the scale, I have known advisors who expected their students to put in a minimum of 70 to 80 hours per week in the laboratory. (Of course, since this is lab work, there's no way to do much of it "from home"—other than reading and writing.) And in my own experience, there are weeks where I barely worked 40 hours, and others where I put in close to 90 (usually right before conferences and when I was racing to finish my thesis).

In the long run, there are very few absolutes when it comes to the career of a graduate student, and you really do need to find your own "path."