My original question was something like "how much productive work can people do in a day?", but even if I got a signed piece of paper from God telling me the exact average, that wouldn't be very useful to individuals except as cover for stopping once they'd reached the average. Some people are clearly capable of doing more than others, the same person has different capacities for different kinds of work.
So instead I'm asking "How do you know when you need a break? When you're done for the day?". My hope is that reading answers will inspire people to recognize tells in themselves they previously hadn't, and this will let them better assess when to push through and when to break.