Atwood doesn't run a rigid clock. She's described a loose shape to her day, mid-morning into the afternoon, that bends around travel and everything else a working author has to do. The constant is that she writes, conditions be damned.

The Direct Answer
Mid-morning into the afternoon, usually wrapping around 4pm.
She described the shape of her day in her How I Write interview, beginning by transcribing the previous day's longhand.

The Shape of the Day

In her telling, the morning is ordinary. Get up, breakfast, coffee, then up to the room where she writes. She rarely faces a blank page. Instead she often starts by transcribing what she handwrote the day before, which eases her back into the work without the cold-start panic of facing nothing. By the time the typing is done she's already inside the book again, and the new pages come easier for it. The session tends to run into the afternoon and ease off around 4pm, though that's a tendency, not a rule she enforces.

If I'm really zipping along on a novel, I'll go into the evening.

Margaret Atwood, How I Write

She's also been honest that she'd like to be more of a morning person than she actually is, and that some days the real writing only kicks in once the clock starts to threaten her. There's a stretch of worrying and circling before she plunges in. The procrastination is part of the picture, not a flaw she's hidden, and it's oddly reassuring coming from one of the most prolific novelists alive. The fix isn't to banish the dawdling. It's to make sure the work still happens after it.

Built to Bend

What sets Atwood apart from the ritual-bound writers is portability. She can write in a lot of places, which matters because she travels constantly for festivals, lectures, and press. A writer who needs one perfect desk, one specific chair, and total silence simply couldn't keep her schedule. She's been described as working across two surfaces, one machine connected for research and another kept offline for the actual drafting, so the internet can't pull her out of the page mid-sentence. The lesson there is quietly modern: the biggest threat to an afternoon of writing is the device you're writing on. For her word target on a good day, see how many words a day Atwood writes, and for the wider view, the full Margaret Atwood writing routine.

What You Can Steal