Thanksgiving (fourth Thursday in November): if you don't have an interview invite by Thanksgiving, start thinking about what you'll do if you need to reapply (this so-called turkey rule was substantiated by Lucca's SDN Survey of 2015 and LizzyM's June 2016 Survey)
Based on my own experience and that of a few people I know (so, n = 5ish) it is very possible to get your first interview invitation after Thanksgiving or even the new year. Of course, that could be a weird anomaly or statistical bottleneck of some kind. If the experts say Thanksgiving, listen to them!
I don't think it's ever too early to start thinking about what you will do if you need to reapply. I lined up a job post-graduation pretty early on in my senior year and it made the process less stressful by eliminating a lot of the uncertainty. And by less stressful, I mean it went from a 10/10 to a 9/10...but hey, that's improvement!
First off you should never panic from a lack of an II. However, if you haven't heard anything by late November (thanksgiving is a good day because it sticks in your mind)
It's time to take a serious look at possible gap year plans and beefing up your application for the next year.
Schools take thousands of applications, reduced to hundreds of II, reduced to a few hundred acceptances and WL, to a hundred of so matriculants. More than 80% of applicants at any individual school, get rejected without II
Since acceptance rates over all for the applicant pool across all schools is about 41%, all applicants should assume that rejection is quite possible. therefore they should spend their glide year in continuing prep, adding to their ECs, revisiting MCAT if need be, etc, all to enhance any possible reapplication.