medical school admissions is random. posters on this forum make rational (and often discouraging) arguments that go down the toilet in the randomness / human factor of admissions. however, the following is almost always true:
when you are complete, there is a delay of 1 day to 2 months before someone even considers inviting you for an interview. the range is random. when you receive the invite, the interview dates are backed up. as an extreme example, in the Albany thread, someone got an II in November for a March interview date! that's a 3-4 month back up! typically, the back up is 2 weeks - 2 months. January is a busy month for interview invites (a good thing) but after that the season has cooled big time. so, you want to interview by Dec. If you are complete by Aug 1, you'll get your interview hopefully by Dec 1 at the latest (typically). that's a good position to be in. nevermind how late schools interview up to - mind how quickly they run out of II!
honestly, for something like volunteering, I would submit the application early and submit an update letter later on.