It is 3 months notice for me (also in Germany) for either side to quit the contract. However, there are no real consequences if I would just leave without notice. Other than my employer having a bad opinion of me afterward of course.

Like, I could just stop showing up and they would fire me for cause without notice, but that matters only if I needed unemployment pay. If I already had a new job that I would like to start immediately, I could just do that.