Actually what does a Doctor need to do in the Academy for 3 or 4 years.
In my country in some cases officers get their degrees in the Academy. McCoy obviously already has his. McCoy in TOS didn't seem to know much about non-human medicine. He had to pick that up on the fly it seemed. So I don't think in TOS they were teaching Doctors much about Space Medicine in the Academy so I'm thinking 3 years is plenty for McCoy.