About two years ago - it is possible that the policy has been loosened since then, but I'm not in human resources.

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.

ah, right, there are two independent regulations. i forgot about Elternzeit which includes a lot of flexibility in adjusting your worktime as needed, whereas the other regulation simply allows you to request a fixed reduction of your worktime. which is a change of your contract, whereas Elternzeit isn't.