Always stop for the day while you still know what will happen next.
“There is a difference between stopping and foundering. To make steady progress, having a daily word-count quota was far less important to Hemingway than making sure he never emptied the well of his imagination. In an October 1935 article in Esquire (Monlogue to the Maestro: A High Seas Letter) Hemingway offers this advice to a young writer:
The best way is always to stop when you are going good and when you know what will happen next. If you do that every day when you are writing a novel you will never be stuck. That is the most valuable thing I can tell you so try to remember it.”