Cal Newport says something to the effect that the median capacity for high-intensity focus is about four hours per day in Deep Work. I think this can bounce around over time for individuals – especially regarding psychological dispositions like bipolar or autism, which proffer an immense capacity for hyper-fixation at the cost of occasional catatonia. Aside from fluctuation, there's remarkable variance between people.

On the high-end, some people can maintain their peak-intensity on a regular basis for ten hours (e.g. John Carmack) without burning out long-term or experiencing short-term crashes. On the low-end, despite a healthy life-style, some people struggle to stay awake during the day (search "you definitely have a thyroid problem" on r/productivity).

What are the deciding factors of this variance, that stake out interventions or practices people can implement to dramatically improve this ability? 

[edit/add: drugs are another frame of cognitive endurance – modafinil can, within hours, bring a person to a level of concentration and energy further than a healthy diet and exercise could perhaps ever take that person. Is there any robust method to achieve modafinil-enhanced performance as a baseline – and without chemical intervention?

I've noticed that general cognitive endurance matters to performance more than personal interest, and that I have high cognitive endurance for some things but at times zero for others, which makes it so that I can spend the entire day doing something that looks like intense work on the outside, but is internally experienced as "just another minute"-style procrastination. This is frustrating, and I've had to drop many classes for fatigue reasons.

It's also interesting that there isn't a specific guide to maintaining high-levels of general cognitive endurance, given its economic value – though high-demand companies and startups already select from a pool of high-endurance people.]