Cartridge break-in is real, but the progress has its ups and downs. It's different for every cartridge but for my ART-9 it actually got dull for a few hours between 8-10 for me. Started opening up again after that.
The most significant break-in I've ever experienced with a cartridge was with a soundsmith. I remember when it opened up around the 50hr mark and my jaw dropped while listening to Random Access Memories. Years later, while my system has well surpassed the quality that I had with the soundsmoth, I'm hoping that the ART-9 does that to me too!