What are the working hours for full time PhD students in Germany?
You are typically paid for 50 or 65% to do work on some project, but that just means that the university graciously allows you to write your dissertation unpaid in the remaining 50 or 35%.

How much you really spent is hugely dependent on the discipline, the department, the advisor, and yourself. The best case scenario would be when there is a huge overlap between the project and your dissertation, both the department and the advisor are very supportive, and you are very disciplined and get a lot of work done in a short period of time. This does happen, but not everybody is that lucky.