Protect thinking time before other people schedule it for you.

Thinking time is uniquely vulnerable because it looks like nothing from the outside. An empty block on a calendar reads as available to everyone who sees it, and other people fill available space in good faith. If you do not claim it deliberately, it is not that someone takes it. It simply never exists.

Put it on the calendar with a real name and defend it the way you would defend a meeting with someone important. An hour a week is enough to begin. What matters is that it exists somewhere other than your intentions.