Look at Fall 2015 where I had 10 courses worth 32 units not counting the project course where the credits are awarded the following semester. call it 35 units. Roughly, a 3 credit course consists of 3 hours of lecture and a 1.5 hour tutorial. The courses I took that have more than 3 credits typically have a lab component that adds an extra hour.
You are correct that I didn't "attend" every single hour of those. But it just further proves my point that just because you're sitting in a room designated in your time table, it doesn't mean that you are inherently working. Instead of going to those classes, I used my time effectively and studied (not necessarily in the classroom) for a normal 8 hour day. When I say useful work, I don't necessarily mean that something is produced. Like you say, sometimes you need to find dead ends before finding the correct path. That's useful. Yes, you can work for 12 hours in one day. It's just not sustainable. A 80 hour work week implies one of two things:
1) You aren't working as much as you think. Go to the gym. Hang out with friends. Enjoy life.
2) You are about to burnout. Stop. Go to the gym. Hang out with friends. Enjoy life.
If you want to keep perpetuating that you need to be in your building for 60-80 hours to be a good grad student, you are directly contributing to the mental health issues seen among grad students.
I also don’t believe for a second that a mere mortal can pay attention to lectures or read papers for 12 hours. Personally after 5 hours of lectures I can’t pay attention anymore, and it’s a similar for paper reading. Most people I know are like that, and frankly I don’t believe you can actually attend 60 hours of lectures a week in university given the scheduling constraints.