How many years of school are required to become a historian?

Practically I would say 16 ( for a BA) to 17+ ( for an MA), but in theory only 12 or less
are actually required. Let me explain:
The answer would depend on how you define "historian", so let me define a historian as one who earns a living by doing historical research and writes papers and books on history. Such a person can be self-employed and no formal credits from schools are required -- there is no license needed for this job. In order to get your papers published you would find it very helpful to have references from established historians, but I think that if your papers are good enough you could get people to read and recommend your work despite your lack of formal credentials.
If you want a job as a curator of a museum or guide to an historical site, you would have to meet whatever requirements the employer would set. In the absence of other credentials such as historical publications, you would likely have to hold a Masters or at least a BA with a record of historical research , in order to get an interview, due to the competition for such jobs.
If you want to be a college professor you would need a PhD. which requires around 7 years of college or 19 years or school counting high school.
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