Not too long, to be honest. It took a lot less time than I thought it would. It helped that I started on a hill and built the structure around the hill and worked from that and dug my floor, so what I've done is I keep extending the walls. Now that I've got my first floor done, it's a lot easier to just keep extending, and I plan on having 5 or 6 floors to it.
The hill used to have a lot of trees on it, which I had to clear away. If you take a look at the last map that Robert posted a few pages ago, you can get an idea of how it looked back then when I was just starting out.
to this from just a few days ago: