It depends heavily on the specialty and individual school. Every school reviews applications and sends invites differently. Some do it all at once, some do it in discrete waves, some do rolling. Also, if you're applying in a specialty with lots of slots, they'll likely issue lots of invitations in multiple rounds. For something like neurosurg, they'll probably only issue one round, since there are so few slots/candidates.
The best way to tell what is going on is look at the interview thread on SDN based on your specialty (or on whatever forum is most popular for your specialty, e.g., uncleharvey for neurosurg). You can also check on last year's interview thread to see when particular schools issued invites, as they'll likely do something similar to what they did last year.
For me, I booked 6 interviews last week, but I think that has nothing to do with my qualities as a candidate and just with the way those school do their invites.
There are some big school that don't even look at the apps until the 15th of this month, so it's way too early to panic.
