Not terribly much time, by the looks of it. For example, look at the list of boards Disney CEO Bob Iger sits on:
https://www.wsj.com/market-data/quotes/DIS/company-people
Publicly traded companies should have their board compensation made public. Iger sits on a lot of philinthropic organizations; I think 501c3's will have that reported to the IRS. But he probably has a day long meeting for each once or twice a year.
Edit: here's Bloomberg Philinthropic's IRS filling (one of the boards Iger sits on). $10.9k per board member. https://projects.propublica.org/nonprofits/organizations/205602483