The first book I wrote incorporated a lot of existing blog posts. There was still a lot of glue to write, holes to patch, and various updates/fixups.
Another book was from scratch and I did spend a fair number of work hours on it--with the OK of management--supplemented by some (but not outrageous) nights and weekends. It probably ended up being like a part-time job for about 6 months plus additional time spent back and forth with the editor--which wasn't actually a lot.