They note that for someone new to such practice (citing, in particular, a child in the early stages of developing an expert-level skill), an hour a day is a reasonable limit. For those familiar with the rigors of such activities, the limit expands to something like four hours, but rarely more. The implication is that once you’ve hit your deep work

Three to four hours a day, five days a week, of uninterrupted and carefully directed concentration, it turns out, can produce a lot of valuable output.