Schools will want to see that you are familiar with the profession and have put some time into getting to know it.
But a direct answer to your question is tough--it is going to depend on the type of shadowing.
For example, 100 hours of standing in the corner, 8 feet away from the patient, with a dentist who never speaks to you, will pale in comparison to 20 hours of being actively involved with a dentist, having the dentist explain all the instruments to you, and having the dentist ask you what treatment plans you can come up with.
100 sounds a whole lot better than 20 hours, but that isn't always the case. It's the quality of hours you should be going for, and those quality experiences should translate over in your interview, regardless of the actual number of hours you shadowed.
I think shadowing is most important for your own *personal* satisfaction, not simply to impress the school (if your academic stats are strong then schools will still admit you in spite of minimal shadowing experience). Before you fork over six figures for dental school you'll want to be as sure as possible that you won't change your mind halfway through. Whether achieving that peace of mind takes one day of observation or a hundred days depends on the individual pre-dent (and the quality of that observation time, as Gavin mentioned).
I shadowed about 30 hours total the summer before I applied. I spent time with my own general dentist, a periodontist, and orthodontist and another gp. Two different days at each practice. during my interviews I talked about my experiences and how the dentist and I discussed whether of not what I saw was a typical day.
I actually shadowed probably less then 5 hours.. I got a letter from that dentist. and I got into 2 out of the 3 schools that I applied for.
Personally, shadowing always felt really ackward for me, I felt like I was in the way and it was really tough for me actually see anything. Some people have dentists in their family or as really good friends, though, so it might be different for them.. It all depends. None of the dental schools that I applied for said anything, so apparently a letter was good enough.
However, I have a bunch of extracurricular stuff, Pre-dent society, I volunteered at the local hospital, etc..