[Pelam]

I'd say anything beyond level 3 is likely already unsustainable for longer periods if you want to avoid stress related ilnesses. Depends on other factors of course.

[Rygian]

Indeed.

The post neglects to mention at which Level one would start saying "No." And I argue it should be Level 3, to avoid going to Level 4.

If I found myself in Level 5, 6, or 7 the only explanation I would find is that I've very badly mismanaged my time for too long.