Effectively the president does not set his own schedule. His scheduler does that.The presidential schedule is one of the most tightly controlled schedules on the planet. And it is scheduled down to the minute months in advance.
The president's time so important enough that anything that does not involve the president making a decision, getting briefed on making a decision or having a meeting related to legislation is handled by someone else. There is a line out the door of people who want/need to see the president (about decisions and legislation), all of these people need to be scheduled, all of these people are important, their issues are important, cancelling on them had better be for a reason other than "I wanted to skip that meeting". Free time does not exist in a presidential schedule, any unscheduled time is buried in reading briefs, approving position papers and all the other work required for a president. Vacations still have daily briefings and priority work.
If you aren't ready to work at least 80 hours a week, on a rigidly controlled schedule that is set by someone else, don't be the president of the US.