Garcia Marquez built his day around the morning and gave up on the rest of it. He wrote from roughly nine until two or three, then closed the desk for good until the next day. The schedule was tied to a simple domestic anchor: the school run.

The Direct Answer
Every morning, about nine to two or three, after taking his kids to school.
He described the hours himself in Peter Stone's 1981 Paris Review interview, The Art of Fiction No. 69.

Nine to Two, Then Done

On a good working day, Garcia Marquez sat down around nine in the morning and worked until two or three in the afternoon. Earlier in his life the window had been a little different, closer to eight until one, and it drifted later as his habits settled. The shape stayed the same. One long morning block, and once it closed, the writing was over for the day.

He was blunt about how little came out of even a full session. On a good day, those five hours might yield a single short paragraph, four or five lines, and he often tore it up the next morning. The slowness wasn't a problem to fix. It was the cost of the kind of sentences he wanted, and he'd rather spend a whole morning earning a few good lines than fill pages he'd have to throw away.

On a good working day, working from nine o'clock in the morning to two or three in the afternoon, the most I can write is a short paragraph of four or five lines, which I usually tear up the next day.

Gabriel Garcia Marquez, Paris Review, 1981

The School Run as a Clock

The morning block had a practical trigger. He took his children to school, and when he came back the house was empty and quiet and the day was his. He'd sit down and write straight through until it was time to bring them home again. By the time lunch came around he felt he'd earned it, and the afternoon was free for the cinema, for friends, for the small errands of a normal life.

That structure mattered more than it looks. The school drop-off gave him a hard start time he couldn't argue with, and the pickup gave him a hard stop. The whole shape of his fiction routine sat inside a parent's ordinary calendar. For the wider picture, see the full Gabriel Garcia Marquez writing routine.

He also insisted he simply could not write later in the day. The morning energy was the only energy that worked for fiction, and once it was spent, pushing harder produced nothing he wanted to keep. So he didn't push. He treated the afternoon as recovery and life rather than a second shift, and he guarded the morning fiercely because it was the only part of the day that did the real work.

What You Can Steal