Prettier was such an awesome project because it already has lot of well-established research, so it came together really quickly. I just got so frustrated that it didn't exist so I sat down, took an existing printer (recast) and started hacking on it.
I started around Dec 1 of 2016 and had something that handled 95% of the common cases by January. So it only took about a month to be close to the initial release. This was mainly because I had a paper (A Prettier Printer) which described the algorithm in full detail and an existing project (recast) to fork and hack which covered a lot of the mundane material.