According to many posters here, the best time ranges from the summer after sophomore to April in the junior year if you do not take a gap year. Incoming freshmen may have a chance to take MCAT in 2014 before the new MCAT. If would ne wise to take care of most pre-req and a few humanity courses such as PSY and SOC.
S1 is taking a course about international human rights. This professor assigns three papers with ~ 40 pages each every week. In addition, every student has to write a response to an article from a foreign journal (on-line version of The Economists in his case). It has been a busy freshmen year for him thanks to the professors. I just asked him to take a practice test (VR from EK) every 2-3 months to gauge any progress and adjust if necessary.
For the preparation of PS and BS, all he can do is to read the chapters in the textbooks before the classes during the semester. He also has to read the research papers from his lab group. There is really no time for preparing MCAT during the school year. Hopefully, all these readings can help easing the future MCAT preparation.