I typically don't wait more than 30-45 minutes before I start asking questions. I went to a doctor one time who always had a waiting room full. When I got there, I asked if everyone was waiting to see him - yep. I just told them to reschedule me right then. Same thing next time. There was no third time.
My time is just as valuable as the doctors, and I have many demands on my time. I don't make people who schedule appointments with me wait and I don't wait long for someone else.