50-100 hours.
anything more is just useless. Plus shadowing is a boring thing to do anyways ESSPECIALLY dental shadowing cause all you do is litterally stand there and watch someone stick thier hands in patients mouth. Sure you can ask "intellegant" questions here and there, but overall very boring process.
However, shadowing different specialties might be cool, I've only shadowed 2 dentists and both were General practice. I assume shadowing an OMFS / ortho / and a few otheres might have been cool.