The ADCOMs are looking for "volunteer" (which, btw, does not have to be pro bono; if you've been working as a dental assitant or something that's great too) experience just becuase they want to know that you've had SOME exposure to the field and aren't just blindly, blithely waltzing in to d-school, lured by promises of high profit margins and normal office hours. The kind of experinece you have - shadowing a general dentist, volunteering at a clinic, working for your endodontist - isn't all that critical, per se; neither is the exact amount of time (as long as it's significant - 5 hours of shadwoing won't impress anyone). The crucial thing is to show the ADCOMs that you know exactly what you're getting yourself into.
Just another point - the amount/type you need might vary by your stats. For example, if you've spectacular stats, then 30 hours of shadowing might be plenty, whereas if your stats are less stellar, many months of varied shadowing/volunteering/research experience might give your app a boost.
This has been just my experience, though, anyone else?