Here's what I did, and it seemed to be plenty:
I had a total of maybe 35-40 hours that I shadowed. I made sure I spent a good amount of time at general practice offices, maybe about 6-8 hours were spent shadowing at the clinics at UofL and UK. Then I shadowed about 3-4 hours each at an Orthodontist, Oral Surgeon, Endodontist, and Prosthodontist.
From what I've heard, they like to see that you at least spent some time at a general practice office, because you'll mainly be doing general dentistry in dental school (for the most part).