When you have fought hard and won? When you have fought hard and lost? When you have fought and see no conceivable outcome?
It depends upon what the break is meant for. If it is for the purpose of resting, at any time you deem fit.
If it is to quit, NEVER!
Breaks aren’t meant to suspend activity indefinitely, but for a moment. Often when in the middle of a break, we allow our attention to hit upon a flash of something that distracts, chasing after emotions and hopes that have no promise to fulfill or accomplish. When your activity isn’t productive in the pursuit of our passions, a break isn’t needed but a train wreck to make you stop so you can resume your break and pursue your passions.
Never let your break become an excuse to stop in your pursuit of dreams.
Take a break if you need too, but never QUIT your dreams!