I've heard of incredibly variable experiences from people working on science PhDs.

I've known maths PhD students who found they could only really do 2-3 hours of truly productive maths in a day, allowing them to spend the rest of the day playing games and suchlike, and after two years their supervisor told them they'd got enough interesting results to graduate.

And I've known chemistry PhD students whose supervisors felt if you're doing a mere 8 hours a day of work you might as well drop out now because you're obviously not serious about graduating, and expected their students to be in the lab tending to long-running experiments on evenings, weekends and even christmas day. And yes, we only give you three years of funding but everyone takes four years to graduate.