well, i only have 50 hours, and i even think that is enough. i think accumulating a good amount of hours is awesome, though, because it shows that you know what you're getting yourself into and that you have seen enough of the field to be able to know you want to be a dentist/go to dental school.
the thing of it is, there are only SO many hours you can sit there and watch the dentist do a lot of the same procedures, especially because since he/she is working inside the mouth, you really can't see much anyway. the dentist i shadowed let me come look over his shoulder many times, and it was still pretty difficult to see everything clearly. if you are thinking of specializing, it might be interesting to observe different specialists on top of a general dentist.
i think the amount of hours you have is perfect. it shows a great commitment, IMO. what did the dentist who posted have to say?