There’s a difference between productive and deep focus.

Even if it’s a factoid, based on personal experience and anecdotal evidence from colleagues, 4h is a good approximation.

I will admit though, that there’s a confounding variable here, that is how much your coworkers value your time. When I worked 4h or 6h workdays, everyone considered my time to carry a premium, so they avoided interrupting me if they could.

I do know people that can work extremely long hours and still be productive but they are quite rare. For a large majority of people a ninety hour work week, would be extremely counter productive at best but most likely dangerous for your mental health.

https://www.popsci.com/work-week-hours-productivity/

I'm curious about the nature of the 90hours per week too.

Is it ~slow paced work, no rushing but constantly walking ideas and attempts ?

Can they endure 90 hours of hard dead ends / frustration / disappointment ? Or is it 90hours of tangible progress (lets include small dead ends that provide nice references for more ideas) and pleasure ?

Newport's book puts the distinction between deep work and shallow work.

Calling up restaurants and trying to get them to switch to your booking software is shallow work. Needing to sit down and program the complete back end of that booking software including testing by Friday is deep work.

You can only do so much deep work before your reserves of energy are depleted.

The assumption that programming is deep work seems like a self-serving prejudice. Which parts of it are automatic will differ from person to person, but I imagine that a substantial majority of programmers will spend a substantial chunk of their programming work doing things that are shallow to them.

One of the challenges of deep work is getting in the zone. Does the book go over that?

If by "zone" you mean "flow," then yes; here's a relevant quote from Deep Work:

> The connection between deep work and flow should be clear: Deep work is an activity well suited to generate a flow state (the phrases used by Csikszentmihalyi to describe what generates flow include notions of stretching your mind to its limits, concentrating, and losing yourself in an activity—all of which also describe deep work). And as we just learned, flow generates happiness. Combining these two ideas we get a powerful argument from psychology in favor of depth. Decades of research stemming from Csikszentmihalyi’s original ESM experiments validate that the act of going deep orders the consciousness in a way that makes life worthwhile."

Newport, Cal. Deep Work: Rules for Focused Success in a Distracted World (p. 85). Grand Central Publishing. Kindle Edition.

Since the OP starts off quoting Cal Newport's book "Deep Work", we need to differentiate between shallow work and deep work.

Running a startup is a whole bunch of shallow work most of the time.

Work where you have to get into "the zone" to get it done, that's not 90 hours per week. Or likely even 19.