[frankbyte]

“There were times when, some weeks ... I haven’t counted exactly, but I would just sort of sleep for a few hours, work, sleep for a few hours, work, seven days a week. Some of those days must have been 120 hours or something nutty.”

Now, Musk said he is “down to 80 or 90” hours of work per week and “it’s pretty manageable.”

https://www.cnbc.com/2018/11/05/elon-musk-on-working-120-hou...

That's the quickest I could find. But yeah, I suppose 20-23h constantly long-term would not be doable. Point is though that many people can and do work even long-term the equavalent of 2 "full-time jobs".