15 or 20 hours is probably plenty unless you find a dentist who actually lets you assist. The purpose of shadowing is just to get an idea of what dentistry is all about. In medicine there is a place for just about everybody, but dentistry really requires a certain type of personality. A dental degree opens a lot of doors, but it is a unique individual who ever really wants to walk through any of them.
There are people in my class who chose dentistry for the money and lifestyle and they are hating life. They hate dentistry; they hate seeing patients, they hate lab work, they hate classes, they hate that nobody outside of school understands what it is they do, they hate getting very little respect for the effort that they put in.
I'm not trying to paint a terrible picture for you. I love dentistry; I honestly get up in the mornings and can't wait to get started when I have a fun case scheduled. I read dental material that isn't assigned because I... gasp... find it interesting. But dentistry is NOT for everyone.
And ironically, the people who are in dentistry only for the money and cushy lifestyle are the most likely to NOT reap those rewards. The people who actually enjoy the profession enough to spend time developing their skills and expanding their repertoire are the ones who become the most successful. People who hate dentistry are simply not going to put in the time needed to become excellent dentists. These people are going to coast by on the minimum, least labor intensive CE they can get away with and never progress much beyond their dental school training.
Anyway, that's probably more than you asked for. The point I was trying to get across before I got sidetracked is to spend your shadowing hours deciding if dentistry is really something you can see yourself doing for the rest of your life. Don't worry too much about figuring out why the dentist is doing what he is doing or the specifics of certain procedures; it really doesn't matter at this point. Spend as much time as it takes to decide if you are cut out to be a dentist. After you've seen enough to make a semi-informed decision there's really not much point in continuing.