I'm building one as well, but I am currently focusing on just the prefabs. I wish to make entirely with custom prefabs. Not that I dislike the ones provided for us, but if I am going to go through all the work I want it to be at least fresh and new for anyone who plays it. So yeah, I can foresee at least 1000 hours of work just in that. Then, you still have to place them and make sure the terrain conforms properly... and that part right there is the big time consumer. We have the means to adjust terrain height on the map, but fine-tuning it to the exact block height is a tricky process. As if just figuring out the RGB value for a certain pixel representing the terrain top-down isn't enough, you have to have this height in the terrain match with the dimensions of your prefab. Then, let's say you get that far, now you might have situations where you adjusted the terrain so much to comply with the prefab, that it now needs to be smoothed out some.
Keep in mind this is only half of it. The other half is the roads, where the same terrain adjustments must be made.