It's totally up to you and how busy the doctor is. During the school year, I only went in once or twice a week, but during winter break/summer, I shadowed almost every day (for a month). It all depended on whether or not I felt like going in and if there were any cool surgeries to watch. As far as for how long, it could be anywhere from a couple of hours to like up to 10 hrs/day, again depending on how long you want to stay and what you want to see.
If a doctor allows a pre-med to shadow, they're most likely pretty flexible and will tell you you can come and go as you wish.