Isaac Asimov ran his writing life the way his father ran the family candy store, open every single day with no exceptions. He woke before dawn, sat down at the typewriter, and stayed there until night. He held that rhythm for decades, and it produced nearly 500 books.

The Direct Answer
He wrote seven days a week, from early morning until roughly ten at night.
Asimov treated writing as a full-time job with no weekends and no holidays off, a discipline he traced straight back to his family's Brooklyn candy store, per his memoir I. Asimov.

A Candy-Store Work Ethic

Asimov grew up above and behind a series of Brooklyn candy stores that his family kept open from six in the morning until one the next morning, seven days a week. Nobody in that house took a day off, and the habit fixed itself in him for life. As an adult he woke around dawn, and by the time most people were sitting down to breakfast he'd already been at the keys for an hour. He worked through lunch with barely a pause, usually a sandwich at the desk, then went back after dinner and typed until roughly ten in the evening.

The setting was deliberately dull. He kept the window shades drawn in his study and worked by artificial light so nothing outside could pull his eyes off the page. A view, to Asimov, was a distraction he couldn't afford. See the full Isaac Asimov writing routine for how meals, mail, and the rest of the day arranged themselves around that desk.

He worked upstairs in an attic office at an electric typewriter, and he kept a spare machine on hand in case the first one ever broke down mid-thought. There was no ceremony to any of it. He'd climb the stairs, sit, and start banging out sentences, watching the words take shape on the page in a way he once described as a little like magic. The furniture was ordinary. What mattered was that the room had exactly one purpose and no competing ones.

I wake at five in the morning. I get to work as early as I can. I work as long as I can. I do this every day of the week, including holidays.

Isaac Asimov, I. Asimov: A Memoir

That schedule barely flexed across forty years of professional writing. Deadlines came and went, success came and went, and the desk stayed the center of gravity. He liked to point out that he owned an alarm clock but never set it, because he woke on his own at the same early hour every morning without being told to.

Why He Never Took a Vacation

Most writers protect their time off. Asimov protected his desk. He didn't take vacations by choice, and on the rare occasions when one was forced on him, he'd try to smuggle work along and keep going. He was frightened of flying and boarded a plane only twice in his entire life. Given the choice, he'd rather be home at the typewriter than anywhere else, and that's usually where he stayed.

None of this came from grim willpower. He genuinely enjoyed it. Writing wasn't the thing he did so he could go do something else. It was the something else, and he arranged his entire life so he'd have more room for it.

What You Can Steal