I'm not sure I trust any self-reported numbers on this kind of stuff. 10 - 20 days per year (but not evenly distributed) is a common figure I see thrown around: https://www.nedcapital.co.uk/how-much-time-does-a-non-execut...
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.