And so you hopefully don’t waste the same amount of time, here are just the conclusions:
Take breaks every 20 to 70 minutes. Finding the right frequency for you may take trial and error. Multiple sources agree that something near 50 minutes of work with 10 minutes of break works well. The Pomodoro technique combines more frequent shorter breaks with infrequent longer breaks, and is commonly used. The longer you go between breaks, the longer your breaks should be.
A break is only effective when you do something different from what you’re doing now. The primary differences that matter are using different areas of the brain (or not trying to utilize your brain during a break), a difference in eye-focus (stop looking at screens if you were, look far away if you were focused on something right in front of you), and a difference in physical activity (move more if you weren’t moving, or stop for a bit if you were).
(Vacations have beneficial effects, but these seem to be limited in scope and duration. I believe this take is missing important nuance.)
If you wish to read all my notes on this topic, they are publicly available here. (Note: In case that website goes down, I do regularly back up my notes (and blog posts) using the services linked to on Archives & Sources.)