I pull it off when you stop being able to see CO2 bubbles and all foam has fallen. Shortly after, the internal temperature will drop 1C or more, and it shows on my stir plate probe. That's usually around the 24 hour mark.
Refractometer indicates over a 70% attunement every time, so I just call it good enough. I'd rather have a longer cold crash so I don't decant off the cells I just worked so hard to create.