After graduating from high school, she would have needed to complete a bachelor's degree somewhere in some pre-med major. Medical school would be four years (two in class and two in rotation) and OTS is three months.

FYI, to give you my credentials, I'm former military and a lawyer, which has the same kind of entry rank and requirements (bachelor's degree, then professional school) as a doctor.

Thank you fanofall. That really helps.

So far, we have:
1965 - Born
1983 - Graduated HS at age 18 (assuming she was born at the beginning of the year)
1987 - Bachelors degree
1991 - Medical degree
???? - Officer Training School - She could have gone anytime after earning her Bachelors, but I'm assuming she would have waited until earning her MD. That way she would have come in as a Captain.
1997 - Assigned to the SGC as a Captain.
Her divorce would have taken place sometime before 1997 as well.

Also, I'm thinking that she would have gone beyond what a usual doctor would have done - in the same way that an ophthalmologist or psychiatrist has more than 4 years of medical schooling. She might have done another rotation in a specialty.

Of course, Janet might have been military before she went to medical school. In that case, my timeline is completely wrong. These gaps are what fanfiction is all about. (g)