I think ~40 hours would prolly be O.K., but try to get as much as you can - not just for the admisssions committee, but for yourself. Just a few days or even weeks of shadowing/volunteering is not going to give you a very accurate picture of what dentistry is all about. Dental school is going to be a huge committment of time and money, and you need to be sure it's really where you want to go.
Actually being in a dental office every day or every couple of days really changes your perspective on things. At least it did for me. In fact, I wasn't even really that interested in dentistry when I started shadowing, I mostly did it cause my school offered elective credit for the experience.
Good Luck.