It’s definitely possible to “work” that much if work consists of being present in the office and nominally working on something or thinking about it. There’s a lot of menial work in most jobs that you can kind of force yourself through for hours at a time.
60 hours a week is easy: just work from 8-6 six days a week.
To get past that, the easy option is to add Sundays. But you do burn out if you work all Sunday, so taking Sunday to rest is a good idea. Let’s say you just work from 6-10pm on Sundays to map out the week and get ready for Monday, that leaves plenty of time to rest.
So now you’re at 64 hours a week, and you want to bump it up. Not too hard: give yourself a dinner break from 6-8 each night, and work again from 8 to midnight M-F. That’s an extra 20 hours, so you’re at 84. And that’s actually not too bad because you have Saturday nights free to socialize and Sunday morning to sleep off your hangover.
Getting past 84 hours is harder, you’ll have to sacrifice Sunday. But if you go ahead and adopt your normal 14h schedule Monday-Saturday you’re already at 84 without working Sunday, and by comparison fitting another 11 hours on Sunday still feels like a little bit of a breather. Maybe you just work 8-7 on Sundays and use the last few hours for some Netflix and go to bed early.
All of this is much easier if you’re not trying to spend 8 hours a day in bed. Suppose you find that 6 hours a day in bed can get you through, now you’ve got an extra 14 hours a week to do thinks like go jogging at 6am or whatever else helps you de-stress.
TLDR; 60 hours a week is relatively easy. 80 hours a week is harder but still manageable. The difficulty ramps as you go from 80-100. 100 hours a week is very hard. Anything over that is unsustainable, but can still be managed in short bursts if you don’t sleep, and we have lots of drugs that can help you avoid sleep.