They tracked musicians, scientists and many others who spend hours of time using their minds and focused on tasks and that is what they found. I know for myself even if I'm in a good writing flow, the characters are speaking to me and I'm just jotting away, 20 minutes maybe 30 at the outside is all I can manage before my mind takes a break of its own. I'm experienced in meditation and I can keep focus in a trance-like meditative state for 15-20 minutes and then my mind breaks and I have to refocus and go again.