Hemingway was a morning writer, and an early one. He started at first light and was usually done before lunch, with the rest of the day left for everything that wasn't writing.

The Direct Answer
Every morning, starting at first light (around 6 a.m.) and finishing by midday.
He wrote in the cool early hours when no one could disturb him, then stopped around noon. Afternoons and evenings were for fishing, reading, and living, which he saw as fuel for the next morning's work.

First Light, Every Day

Hemingway laid out his schedule plainly in the 1958 Paris Review interview. He wrote first thing, as soon as there was light to see by, usually around six in the morning. The early start wasn't about willpower or productivity culture. It was about conditions. The house was silent, the air was cool, and his mind was clear before the day had a chance to crowd in.

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.

Ernest Hemingway, Paris Review, 1958

He worked through the morning at a steady pace, standing at his board, and typically finished by noon. A morning's work was around 500 words, sometimes more. Then he stopped, deliberately, while he still knew what would happen next.

Why Mornings Worked for Him

The early hours gave Hemingway two things he prized: solitude and a fresh head. He believed the writing should happen before the world got loud and before fatigue set in. By starting at dawn, he spent his sharpest hours on the page and his dullest hours on everything else.

Just as important was what he did after. Hemingway didn't write all day. He fished, he read, he saw people, and he let the afternoon refill the well. The morning schedule only works if you respect the off-hours as part of the method rather than time stolen from it.

For the complete picture of how Hemingway worked, including his desk, his pencils, his revision habits, and the famous theory of omission, see the full Ernest Hemingway writing routine page.

What You Can Steal