It looks like becoming a PhD in Archaeology can be as little as 6 years (about 15 % of candidates) to about 9 years.
All of this is levels of class instruction and related field work. I don't know those specifics but these curriculums are probably easily found.
In most fields, one usually needs 'some' Bachelors, so about 4 normal years, a Masters which is 2 or less and then I'm not even sure about the actual Doctorate needs.
I think it is easy to see that Susan is probably much more intelligent than the average Marvel super character, male or female.
So she could easily have finished this course work as that 15 % or better.