Ferdinand Vogelgsang (former member of the TUM Graduate Council) was interviewed by the Bayrische Rundfunk about this doctoral studies.
One thing is needed for doctoral studies: Passion!
On average, a doctoral thesis takes four and a half years in Germany. What you should know, if you are thinking about doing your doctorate. The video (in German only) with Ferdinand Vogelgsang (Doctoral Representative at the Chemistry department and former member of the TUM Graduate Council) can be found here.