90 minutes at a time.

A lot can be done, 90 minutes at a time.

A lot less would be done, in let’s say, 6 hours at a time. Our brain can’t process that much time. It won’t initiate the process that would help us focus.

90 minutes allow us to warm up, focus, get distracted, come back and have something tangible at the end of the session.

It’s equally important to take at least a 30-minute break (away from any screen) between two 90-minute focus sessions.

Three such sessions in a day can be plenty for a good day's work.