The really strong protections are the first three years after a child is born - you have to be allowed to work anywhere from 15 to 30 hours/week, and be allowed to change it with a few weeks' notice ("Elternzeit" - covers both parental leave and this part time working arrangement).
I had to give a reason for wanting to do the next five years part-time, but "child" resulted in no further questions. I had to commit to only being allowed to work part time, and for the weekly hours I requested for those five years; my employer could choose to accept a request to change them or to go back to full time before the five years is up, but they're not legally obligated to.