Over the first 15+ years of my career as an engineer and engineering manager, I've repeatedly surveyed colleagues and friends in engineering roles about this question. I've asked it of people I consider low-performing, all the way to folks I'd consider 10x developers.

I have never met a single software developer who, when pressed to give an honest assessment and when "working" is defined along the lines of "actively defining, coding, or debugging a feature", self-identifies as working more than 50% of the time. There are occasional stretches of 12+ hour working sessions, but they are very rare.

I'd say I've asked this of over a hundred people.

This isn't to say these people only do 4 hours of work a day. Often, peers have described how they hit a wall at work, go home, and then work on personal projects in the evening.

I've come to the conclusion that is generally impossible to do mentally intensive tasks for more than 4 continuous hours over the long term.