In Ericsson’s seminal 1993 paper on the topic, titled “The Role of Deliberate Practice in the Acquisition of Expert Performance,” he dedicates a sect… - Cal Newport

Ericsson notes that for a novice, somewhere around an hour a day of intense concentration seems to be a limit, while for experts this number can expand to as many as four hours — but rarely more.

You have a finite amount of willpower that becomes depleted as you use it. Your will, in other words, is not a manifestation of your character that you can deploy without limit; it’s instead like a muscle that tires. This is why the subjects in the Hofmann and Baumeister study had such a hard time fighting desires — over time these distractions drained their finite pool of willpower until they could no longer resist. The same will happen to you, regardless of your intentions — unless, that is, you’re smart about your habits.

You need to get good in order to get good things in your working life.

This is what ultimately matters: where you end up, not the speed at which you get there, or the number of people you impress with your jittery busyness along the way.