How long does it take to receive feedback for Junior Professorship (W1) in Germany after interview?
Christian Clason's comment is basically correct. The hiring process for W1 is the same as W2 and W3 and reports are collected after the interviews. So the time from interview to notification is not short. However, it don't need to be six month. 

What usually happens after 
the interviews is that the committee solicites reports and the referees get about six to ten weeks for their reports. Then the committee meets again and forms its recommendation. This is the first point at which you may be told your suggested listing, but this is unofficial and at some places even against the rules. The next steps are that a number of committees have to approve the list (can be something between two and four) and only after the head of the university has approved the list, the first place may be informed officially. This whole process may well take six months, but it can be less, but do not expect less than six weeks between interviews and unofficial notification and not less then three months between interview and official offer.