I’m constantly amazed at how, in the past, the idea that four or five hours or really focused work was a solid day for the thinker or artist was the conventional wisdom– and how thoroughly we’ve forgotten it.
Lest one think this is the kind of advice that you’d only hear from artistic types like Oscar Wilde, or independently wealthy savants like Charles Darwin, here’s William Osler making the same argument, in A Way of Life: An Address to Yale Students, Sunday evening, April 20th, 1913 (available on the Internet Archive):
Realize that you have sixteen waking hours, three or four of which at least should be devoted to making a silent conquest of your mental machinery. Concentration, by which is grown gradually the power to wrestle successfully with any subject, is the secret of successful study. No mind however dull can escape the brightness that comes from steady application…. A few hours out of the sixteen will suffice, only let them be hours of daily dedication in routine, in order and in system, and day by day you will gain in power over the mental mechanism…. Shut close in hour-tight compartments, with the mind directed intensely upon the subject in hand, you will acquire the capacity to do more and more, you will get into training; and once the mental habit is established, you are safe for life…
Four or five hours daily it is not much to ask; but one day must tell another, one week certify another, one month bear witness to another of the same story, and you will acquire a habit by which the one-talent man will earn a high interest, and by which the ten-talent man may at least save his capital.
In his case, Osler matches the idea of four solid hours with being focused and systematic with your time: the more you can control your day, the more you clear away the time necessary for really deep thinking.