For Elon Musk, the difference between a "manageable" work schedule and an "insane" one is somewhere between 80 and 120 hours of work per week.

"You're gonna go a little bonkers if you work 120 hours a week," Musk told Recode's Kara Swisher in an interview on Wednesday at Tesla's Palo Alto, California headquarters.

"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."

Musk said while "80 is pretty sustainable," the increase in suffering when you work more than that is dramatic. "The pain level for hours increases exponentially. It's like nonlinear above 80," Musk said, according to a transcript.

Still, 80 hours a week is over 10 hours more than the average CEO of a multibillion-dollar company works.

A study published in June by Harvard Business Review found CEOs worked an average of 62.5 hours a week. On average, the CEOs monitored for the study worked 9.7 hours per weekday, which totals just 48.5 hours per work week. They also worked 79 percent of weekend days at an average of 3.9 hours daily, and 70 percent of vacation days with an average of 2.4 hours on those days.

For the study, Harvard professors Michael Porter and Nitin Nohria, tracked how 27 CEOs of companies with an average annual revenue of $13.1 billion spent their days in 15 minutes increments, 24 hours a day, seven days a week for three months.

As for Musk, he has attributed some of his recent controversial behavior to his work schedule.