24 hours after they sink (for those keeping score at home, that is 36 hours after placing the seeds in the cup initially), I check to see if any have germinated.

If the seeds have cracked and I can see the white "tap root", I consider that a successful germination. I do not necessarily wait for the root to grow. But I have and that is one thing that is nice about this, there aren't many time constraints with this method.