It is clear that more work is done if you go from 1 day to 5 or 1 hour to 8, but that still includes plenty of time for rest.

Does the same hold for going from 5 days to 7 days or 8 hours to 16 hours?

A reasonable person should agree at some point adding more time will make people less productive due to lack of breaks, rest, etc.

Is it obvious where the line is? if so, where?