I think it depends on two things:
1. How well do you get along with the doc?
2. What area of medicine is it in?
For example, one doc I shadowed was in EM and we got along so well that I did it for 3-4 months at about 1-2 times per week for about 5-7 hours each time. There were always tons of things going on, so when he was busy with paperwork, I would find something else to do (like talk to patients, etc). If you're shadowing a family doc, you may not have as much stuff to see. Start shadowing and get a feel for how long you think it would take you to really understand what's going on. If you really love it, and the doc doesn't mind, keep going!