Being a non-executive board member often involves very little time or attention, it's mostly lending your name and credibility to the organization. Consider it an American form of royal patronage[0]
10-20 days a year is in the same ballpark and is still a lot. I quoted 6 work weeks (40 hours per day at 5 days per week) which is 30 days a year, not that much higher than your stated 10-20 days.
The article I linked specifically mentioned that private company boards demand less time than public company boards, around 200 hours, roughly 25 work days.
Any of these figures is pretty close to my entire yearly PTO allocation as an individual contributor.
Nike consistently has 5 board meetings per year, so Tim Cook is taking time out of running the second most valuable company in the world to do that, for some reason.
[0] https://www.cosmopolitan.com/entertainment/celebs/a63431790/...