Just more anecdata, but this jives with me. I keep time logs. I have "maximal mental effort" (MME) and "everything else." For me it's about scope and depth: MME is about how much I can integrate and bring to bear on what I'm doing, like my capacity to coherently integrate citations into my writing. So perhaps it's how large and long you can subconsciously sustain "useful potential inputs" to conscious working memory. Monkey coding I can do, for sure, many hours a day. But coding at my maximum ability is, again, 2-4 hours per day.