Personally I can't do more than 2-3 hours of genuine "deep work" per day. That is high quality, focused, immersed thinking on a difficult problem and writing code. IMO people who work a large number of hours each week get cognitively slowed. They can't tell they've slowed down (partly because they don't have a control to compare against) and so think that if you do 80 hours per week you'll do twice as much as if you…
But yeah, my "real work" limit for a day's probably like 4hrs tops in any kind of sustained way. I've rarely seen a job that can manage to keep even that much real work slung my way on a regular basis, though. And at this point I provide 90+% of my value just by knowing the right thing or having the right insight, and communicating that info. That top-of-the-value-pyramid work is maybe a handful of minutes a week, on a good week. Most of my salary's justified by a few hours a year—it's just hard to know which hours it'll be.