Most of the settings I've created prior to becoming a publisher, I had a kernel of an idea, created a map, laid out the basic local sovereignties, and the politics going on between those sovereignties, over the course of a weekend. I'll spend the next few days creating the fundamentals of one sovereignty intended as the PCs home, or starting base - then develop over time, as needed on the way. Now I have settings I've designed that I never played, but I intended to one day more fully develop then publish, but the bulk sits on a shelf for now, as other projects have gotten in the way, and haven't needed to look at it for awhile. But as a GM, building a setting for a game, but not with any serious intentions of publishing - I run them all. I've never developed a personal setting that wasn't played in, even if for just a handful of sessions (life got in the way of a campaign).
Now the Kaidan setting of Japanese Horror (PFRPG), published as an imprint under Rite Publishing, that I am the developer, cartographer, illustrator, page layout, and share in copyright, that development took about 3 months on my part, before I presented what I had to Steven Russell of Rite Publishing (RIP), whom I had done some commission work previously. We first created the introductory trilogy of modules of 5th - 7th, called the Curse of the Golden Spear with The Gift, Dim Spirit and Dark Path (I created detailed outlines of the first two modules, and an ad hoc one for third module, which I wasn't sure how the first 2 modules might alter how the 3rd is played). I created a series of in depth papers on specific subjects like Samurai and the Bushi, Yakuza, the Social Castes, the Religions - written like term papers, and provided to the authors assigned to each sub-project, though Jonathan McAnulty did the bulk of the writing. Then I served as technical advisor, and sometimes made recommended alterations to given archetype abilities, but I mostly steered the fluff, moreso than the crunch. Steve funded it via his own inhouse Patron system (like his own Kickstarter), and it took about 6 months from funding to release the products. Of course in this case, it wasn't me alone in the creation, and would certainly have taken me a year or more to have accomplished the same on my own.
Considering a total of 16 products were developed between 2010 and 2017, that's 7 years it took to develop it all, granted that the publisher, and myself were working on other projects, for other publications at the same time, so that's not 7 years at 100% work it, so it might have ultimately taken 3 years of actual time dedicated to such an endeavor. Also consider in that same 7 years, I created over 1,100 maps on commission for other publishers.