Don't want to speak for anyone else but I have heard through the grapevine that having to do externships for a considerable number of weeks is a pretty recent development (last 5-10 years). People who are currently practicing oral surgeons, from what I have heard (again, through the grapevine and don't know if this is true), did not have to do as many externships as applicants now have to do.
Also, different schools have different break schedules and different amounts of time you are allowed to miss clinic, so it would not surprise me if there is a large amount in variation of time that people get to extern from school to school.