In Munich, the legal requirement is to register within two weeks, but the waiting time to schedule an appointment is more than two weeks. Luckily, no-one cares. Some of my Erasmus classmates did not register at all.
I think this is uniform across Germany, and I think the legal requirement might be either be registered or have an appointment to be registered, similar to other legally required appointments, which (from my understanding) causes issues regarding endless-reschedule loopholes.
It took me over 3 months to register (which is shocking in itself) - perhaps things are not so dire in Munich