The best time to take a break is when you think you do not have the time for it at all.
Do not get this wrong: This is not meant to encourage breaks all over the day, everyday.
But often times we forget to take a deep breath every once in a while when things get busy. But that is exactly the time we need some break time.
Give yourself the freedom to take those breaks in busy times. It will give you back your focus and determination and that is what you ultimately need.