blankguy, sorry, but you are completely missing the entire point behind this process!
The shadowing/research should be done so you can participate in the field and get a feel for how things happen.
Shadowing lets you see procedures, as well as how an office is run.
Saying you should do XXX hours a month or per week is absolutely absurd. People with 1000 hours of shadowing might not get accepted to dental school. Likewise, I'm sure many people with ZERO hours get accepted all the time.
YOU should be the factor behind all of this. It's so YOU can learn. We can't possibly give you a valid answer because none of us know you, your schedule, or how much background you already have with dentistry.
Hell, I shadowed some and didn't even list it on my application.
So yeah, if you want to move on to another dentist after the first then you probably should! Go see a perio, or a pedo, or an ortho!
I'm shadowing right now in the hospital and I find hospital dentistry fascinating! That's right, I'm already in dental school and STILL shadowing (it's about me learning!).