You write until you come to a place where you still have your juice and know what will happen next and you stop and try to live through until the next day when you hit it again.
Here is an interview conducted by George Plimpton on 1954:
[Hemingway] keeps track of his daily progress—“so as not to kid myself”—on a large chart made out of the side of a cardboard packing case and set up against the wall under the nose of a mounted gazelle head. The numbers on the chart showing the daily output of words differ from 450, 575, 462, 1250, back to 512, the higher figures on days Hemingway puts in extra work so he won’t feel guilty spending the following day fishing on the Gulf Stream.
— George Plimpton for the Paris Review, issue 18, Spring 1958
And here is Hemingway’s daily practice in his own words:
When I am working on a book or a story I write every morning as soon after first light as possible. There is no one to disturb you and it is cool or cold and you come to your work and warm as you write. You read what you have written and, as you always stop when you know what is going to happen next, you go on from there. You write until you come to a place where you still have your juice and know what will happen next and you stop and try to live through until the next day when you hit it again. You have started at six in the morning, say, and may go on until noon or be through before that. When you stop you are as empty, and at the same time never empty but filling, as when you have made love to someone you love. Nothing can hurt you, nothing can happen, nothing means anything until the next day when you do it again. It is the wait until the next day that is hard to get through.
— Ernest Hemingway, The Art of Fiction No. 21