The story that got Shirley Jackson more hate mail than any writer bargained for came together in a single sitting. She said so herself, and the manuscript backs up most of it.

The Direct Answer
She wrote it in one morning, start to finish, with almost no revision.
Her own account in Biography of a Story (1968) describes a first draft that ran nearly word for word into print.

The Morning It Came

On a warm June morning in 1948, Jackson pushed her daughter up the steep hill to her house in North Bennington, Vermont, the stroller loaded with the day's groceries. The idea for the story arrived somewhere on that climb. She parked the baby in the playpen, put the frozen vegetables away, and sat down to write. By the time she read it back, she felt she was done.

Writing the story, I found that it went quickly and easily, moving from beginning to end without pause.

Shirley Jackson, Biography of a Story

She sent the draft to her agent the next day, almost word for word what she'd typed that morning. The New Yorker bought it within about a week and asked for a single change, shifting the date in the story so it matched the issue it would run in. The magazine published it on June 26, 1948.

The Part She Left Out

That's the tidy version, and Jackson told it often at readings. Her surviving papers complicate it a little. Scholars who've gone through her drafts note that New Yorker editors were weighing in earlier that spring, so the jump from first sitting to finished galley wasn't quite as frictionless as the lecture makes it sound. The heart of the claim still holds. The bones of 'The Lottery' really did land in one fast draft, and she barely touched them afterward.

For the wider picture of how she balanced fiction with a full house, see the full Shirley Jackson writing routine.

What You Can Steal