I hate to disagree, but one tooth, two teeth probably a lot of teeth can be off retarded before things start to hit. The other way (advanced) things are different. Less than one tooth might cause interference.
The natural way for the two shafts to fail is in the retard direction. The motor will die from the bad timing before it can hurt itself. Unfortunately engines don't just stop. Failures usually cause large timing changes with each rotation which eventually puts the relationship in advance which is where the problem arises.