Digital Foundry: How long did this take to develop?
Piotr Chrzanowski: Over a year now. Well it depends on whether you want to add the business stuff, but then I would say it's around 12 months at this point.
SOURCE
The full interview is great, and has a lot of details about the kinds of challenges the team faced getting this to happen (RAM was, predictably, an issue, and getting Novigrad to run on Switch was a bit of a pain), so make sure to read the full thing.
But one year for the port is mad impressive. Some technical wizardry at work here.