Most of the poems Plath is remembered for arrived in a matter of weeks. Between late September and December 1962 she wrote at a pace she had never reached before and would not live to repeat, finishing the bulk of the collection that became Ariel.

The Direct Answer
In a concentrated predawn burst in the autumn of 1962, often a poem a day.
She wrote them at Court Green, her farmhouse in Devon, in her own account in an October 1962 letter to her mother.

The Burst of 1962

By that autumn Plath's marriage to Ted Hughes had broken down, and she was alone at Court Green, their house in North Tawton, Devon, with two children under three. What came next was the most concentrated stretch of writing in her life. In October alone she produced around twenty-five poems, among them Lady Lazarus, Daddy, Nick and the Candlestick, and the title poem, Ariel, written on the morning of her thirtieth birthday.

She was not inching through drafts and polishing for weeks. She was completing whole poems in a single early morning. In a letter to her mother dated October 12, 1962, she described the pace without any false modesty.

Every morning, when my sleeping pill wears off, I am up about five, in my study with coffee, writing like mad, have managed a poem a day before breakfast.

Sylvia Plath, letter to Aurelia Plath, October 12, 1962

That rhythm held for weeks. Some mornings brought two poems. The famous pair Ariel and Poppies in October both came on October 27, her birthday, before the children were up.

For a sense of scale, her first collection, The Colossus, had taken years to gather. The Ariel poems overtook it in a handful of weeks. She knew the work was different, too. In her letters that month she said she was writing the best poems of her life and that they would make her name, and she turned out to be right, though she did not live to see it.

Speed on the Surface, Years Underneath

The poems came fast, but the ground under them had been worked for a long time. Plath had written seriously since childhood, filling notebooks, drafting hundreds of poems, and drilling herself in strict meter and rhyme. She had spent years, in her own words, learning that everything in life is writable. When the Ariel voice finally broke open, it moved quickly because the craft was already in her hands and only the permission was new.

It helps to know what she was writing about. Many of the poems draw on the collapse of her marriage, her father's early death, and her own fierce inner weather. She was not writing to a market or a deadline. She was turning the hardest year of her life into some of the sharpest poems in the language, and doing it before breakfast.

The physical drafts back this up. Plath often wrote the Ariel poems on the backs of earlier manuscripts and on pink Smith College memorandum paper, working in the same predawn hours day after day through October and November. The poems were revised, but they were revised at speed, in tight sequence, by a writer who finally knew exactly what she was doing.

See the full Sylvia Plath writing routine for the study, the schedule, and the setting behind the burst.

What You Can Steal