OSU requires a minimum of 20 hours of shadowing. Of course any amount of shadowing you are able to work in is better than none at all but I would aim for at least 20 hours. It's very doable. I suggest that you try to dedicate a few days to nothing but shadowing in which you are there from open to close. Doing so will give you a more complete picture of how a dental office runs on a daily basis and you'll be exposed to a more complete spectrum of patients and assorted ailments.