Dennis M. Dimitri, MD, a family physician, runs a pretty unusual office. Few appointments are accepted in advance. Instead, patients call in the morning and are assigned a time slot later that day. Some patients walk in without calling ahead. The outcome of this lack of advance planning? No one has to spend weeks trying to wrangle an appointment, and once patients arrive, they rarely wait more than a few minutes for the doctor. That might seem mind-boggling to those who have cooled their heels for 30 minutes, or an hour, in their doctor’s reception room — after waiting weeks to get the appointment in the first place. It takes an average of 63 days in the Boston area to get an appointment with a family physician, according to a 2009 survey by physician recruiters Merritt Hawkins & Associates, and 20 days on average for the nation as a whole.