I am doing post-bac at state school, U of Arizona, and actually decided to just stay a bit longer and get a second bachelor's degree in Biology. I'm done in May and will have been here two years since May 2001. I did the year of General Chem my first summer and have had 2 semesters with 4 classes, and two semesters with 3 classes since then. My first degree was in History and I had none of the requirements done when I started, though I had AP Bio which let me start in upper division bio classes. My activities here have been focused solely on med school since I did a lot of other stuff in college the first time around. I've done volunteering in clinical settings and did 3 semesters of independent research. This past summer I did research at another university on an NIH sponsored fellowship program. I actually think doing the full second degree opened a lot of doors in research that would have been harder to do were I doing the minimum requirements. I will have a lag year this coming year while I apply. Had I planned things a bit better I would have applied for 2003, so that is something to think about when you organize your program. Though I think time off is usually a good thing.
If you just do the bare minimum I think you could do it all one summer and one academic year, though it will be lots of tough classes at once. I know here they advise post-bacs to do a semester of Biochemistry and usually Genetics or Cell or Molecular Bio in addition to the minimum. I think doing this at a state school has been great in terms of the access to research opportunities and being in a lower stress environment then exists at some of the formal programs from what I hear. The advising is not the greatest here though so that is also something to consider. If you have any more questions feel free to post them or PM me. Good luck 🙂