Both at work and in other professional activities, a number of workshops, team meetings, etc. I've been in, we've all pretty much collectively decided that three to four hours a day is about the most we can productively handle. We've done some combination of just paring back material and smearing things over more days.

Not that professionals necessarily have the greatest attention spans; I've only somewhat joked that I couldn't deal with all-day high school classes today even in-person. But all-day Zoom seems totally undoable for kids.