When I shadowed I went to the dentist with the times that I was available (in my case, the month of July last summer). He looked through his July schedule and just jotted down the dates that I could be there (he could tell whether or not based on the names of the people if they would mind having someone watch). If something "real cool" came up, like a whole mouth make over, then he asked me if I wanted to see that too. Otherwise, I witnessed mostly crowns, veneers, fillings, whitenings, and implant work. If he is flexible just start naming off dates and see what he thinks of them.
All in all I wanted to see as much as I could so in the end I got in 70+ hours. I think it is really about your schedules.