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.
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.