Zadie Smith has described her writing schedule in enough interviews that a fairly precise picture exists: two protected days a week, a library desk, and the internet switched off. It's built around her teaching job, not around some fantasy of unlimited creative time.

The Direct Answer
Two full days a week, usually Thursdays and Fridays, writing from about 9am to 2:30pm in a library, with internet access disabled.
Detailed in her interview for The Gentlewoman, issue 14 (2016).

Two Days a Week, Built Around Teaching

Smith has taught creative writing at NYU since joining the faculty in 2009. Her writing time gets built into whatever the teaching schedule leaves open, which she's described as Thursdays and Fridays, from around 9am until 2:30pm, timed to end before her children get home from school. That's not much time by the standards of some novelists, but it's fixed and it repeats, which matters more than the total hours.

She's said she has always written in libraries, even before she had children or a teaching job to build a schedule around. For a stretch she had an office at NYU that had belonged to E. L. Doctorow, who taught there until his death in 2015. It didn't last as a solution. Students started finding her there between classes, which defeated the point of leaving the house in the first place.

I have absolutely given up on the idea of peace and quiet as being necessary to writing. I work in the time I have.

Zadie Smith, interview with The Gentlewoman, issue 14 (2016)

No Internet, White Noise, and a Four-Hour Limit

During writing sessions Smith disables her internet connection, and she's talked about using blocking software to make sure the ban holds. In the acknowledgments of NW (2012), she thanked both Freedom and SelfControl for, in her phrasing, 'creating the time.' She's also mentioned working against white noise to manage a shared or open room, and she's said she stays off social media entirely rather than trying to moderate her own use of it.

She's put a rough ceiling on how long a session stays useful, too. At a public conversation with Jeffrey Eugenides at the New Yorker Festival in 2016, she said her own work tends to go bad after around four hours at the desk, well short of the six-to-eight-hour stretches some novelists claim. For how this fits with the rest of her process, see the full Zadie Smith writing routine.

What You Can Steal