It takes between 4 weeks and several years...
Ours should be done by the end of January. I have seen several "Fertigteilhäuser" near our place, but somehow that approach never really appealed to me, so we went with the traditional brick-and-mortar approach. Also, while the frame of the house is done quickly if you build it from wooden prefabricated parts, it still takes quite some time to do all the inside work, cables, heating and so on.
Building with Cellar also eats up a lot of time, especially if it has to be watertight like ours...
We'll move in around the end of February if everything goes according to plan and at the moment it looks like we might manage that deadline. The original deadline was mid-december, but that was pretty unlikely anyway...