Actually, take a 15-minute break between 90-minute cycles and 5-minute breaks within the 90-minute cycles.
It is on page 8-6 of the first edition. Look up study-skills in the index of the other editions. (I only have the first at home, here.)
It might be good for everyone to review it now. A key element is making an agreement with your mind that you will study for the next 20 minutes with full alertness and effortless concentration.
Then, no matter how much you are in the flow state, take that break after 20 minutes. You've made the agreement, you got full alertness and effortless concentration, now take the break. You'll find yourself more successful in the future when you follow this pattern all of the time. If not, you might not get the full alertness, etc., in the future, because a part of you will know you do not adhere to the 20-minute part of the agreement.