Most master writers come with a famous routine, a ritual, a set of daily hours. Christie produced more books than almost anyone and had none of that. Her routine was essentially to not have one.

The Direct Answer
She had no fixed routine and wrote in spells and bursts.
By her own account in her Autobiography, writing was a task she fit around ordinary life rather than a daily ritual at set hours.

Writing in Spells and Bursts

Plenty of writers swear by the same desk at the same hour. Christie didn't run her life that way. Recalling her early married years, she wrote that she had little memory of the books from that period because she was enjoying ordinary life so much. 'Writing was a task which I performed in spells and bursts,' she said. When a book was due, she got it done in a concentrated stretch, sometimes shutting herself away in a hotel for a fortnight to finish, then she set it aside and went back to living. There was no sense of a daily quota hanging over her, no guilt on the days she didn't write.

It worked because the heavy lifting happened in her head. She had already plotted the thing while walking, bathing, or doing chores, so a writing spell was mostly transcription. A 1950s novel might take two months to draft and another to revise, according to her family. That kind of speed is only possible when the puzzle is already solved before you sit down, which is exactly why the missing routine never slowed her output. The thinking ran on its own schedule, and the writing simply caught up.

Occupation: Married Woman

Christie's lack of routine ran deeper than a missing alarm clock. For a long time she didn't even file herself under 'writer.' She admitted that even after publishing ten books she didn't think of herself as a real author, a bona fide one. On forms that asked for her occupation, it never occurred to her to put anything other than 'married woman.' The writing was for entertainment, hers and the reader's, and she refused to dress it up as a grand career. Even landslides of royalties didn't change how she saw herself.

I was a married woman, that was my status, and that was my occupation.

Agatha Christie, An Autobiography

That attitude is half the reason the routine stayed loose. If you don't see yourself as a Professional Writer with a Process, you don't build a cathedral of rituals around the work. You just do it when it needs doing. There's a freedom in that which is easy to envy. No anxiety about whether today's word count measured up, no superstition about a lucky pen or a particular chair, just a clear-headed sense that the book would get written when she set herself to write it. You can read how the rest of it fit together on the full Agatha Christie writing routine page.

What You Can Steal