The best thing I've seen around this issue is that burnout is caused not by working too much (within reason) but by working on things which cause a conflict wit

The best thing I've seen around this issue is that burnout is caused not by working too much (within reason) but by working on things which cause a conflict with your values. Those values might be anything - if you value using your intelligence to solve hard problems but work on trivial nonsense (perceived or real) then you may burn out working 20 hours a week. In other words: hours worked don't really correlate to the kind of deep depression-style burnout that prevents people from working for years. There is a burnout caused by working too much, but that's more of a standard chronic stress response, and can be solved simply by reducing the workload, whereas value-mismatch problems will persist even if you reduce the workload. Value-mismatch burnout requires major changes in your life to recover, basically.