DEFINITELY apply early (beginning of January, if you can). The administrative staff is fairly small, and once applications start rolling in during March, the wait time will be high (you might not hear till May or June), you will be stressed, and your chances of getting in are lower. If you apply in January, your application should be looked at by the end of the month or beginning of February, and you could get a response by the end of February. People are accepted all throughout the summer (until first day of orientation). Admission is fairly competitive. I just started the SMP a few weeks ago, and the numbers are something like 900 applied, 160 are in the program, average GPA 3.3, MCAT 31.4 (don't quote me, but that's what I remember). I definitely encourage you to apply if your GPA is >3.0 and your MCAT is >27, as in my opinion, this is the best SMP program out there.