Didion didn't worship a sunrise start the way some writers do. The fixed point in her day came in the evening, one hour alone before dinner, drink in hand, reading back what she'd written.

The Direct Answer
Her day was built around one hour alone before dinner.
She used that hour, with a drink, to go over the day's pages, per her 1978 Paris Review interview.

The Hour That Mattered Most

Asked about her writing rituals, Didion went straight to the evening. She said the single most important thing was an hour alone before dinner, with a drink, to read over what she'd done that day. She couldn't do it in the late afternoon because she was still too close to the work. The drink helped, she said, because it pulled her back from the pages enough to see them.

During that hour she'd take things out and put other things in. Then she'd start the next morning by redoing all of the previous day's work, following the notes she'd made over her evening drink. The afternoon was for producing pages. The evening was for judging them. Morning picked up where the evening left off.

I need an hour alone before dinner, with a drink, to go over what I've done that day.

Joan Didion, The Paris Review, 1978

Why She Guarded It

When a book was really moving, Didion didn't want to go out or have people over for dinner, because that meant losing the hour. She was blunt about the cost. If she skipped it and woke up the next day with bad pages and nowhere to go, she said, she was in low spirits. The ritual wasn't a luxury. It was the thing that kept the next day from collapsing.

There's a second piece of her schedule worth knowing. Near the end of a book, she needed to sleep in the same room with it. That's part of why she went home to Sacramento to finish things, where nobody cared whether she appeared or not and she could get up and start typing. Somehow, she said, the book doesn't leave you when you're asleep right next to it. For more, see the full Joan Didion writing routine.

What You Can Steal