I remember in particular a duscussion about music students practicing for an optimum, or maybe maximum, number of hours per day. I'm in math and there are a number of people in math who also say four is the limit; a quick google found this for example https://theweek.com/articles/696644/why-should-work-4-hours-.... FWIW.
Also well-konwn are the first two paragraphs of Littlewood's Miscellany, https://www.gwern.net/docs/math/1986-littlewood-littlewoodsm..., p 189.