Rolling admissions committees often look at batches of apps after a preliminary review. They then pick some out of the batch and invite for interviews, the ones not invited are rolled into the next batch until either they stand out enough against the rest for an interview or the end comes. This is how a few schools I've talked to described it at least.
However, if you apply early and by January you haven't gotten interview invites at a school that interviews starting in sept, in my opinion you can pretty much guess you're screwed 😉
Just my opinion though. And all this depends on the school, of course. Some send rejection out very quickly.