Look around at this site as well as mdapplicants pages from previous cycles. You'll see a lot of information about the dates people received interviews.
On another thread it was mentioned (by a med school admissions committee member) that late January is when you shouldn't expect any more good news, so you've got over a month until you're unlikely to get more interviews. In addition, it seems like some schools will continue interviewing and sending invites well past that date, so depending on the schools you've applied to you could be hearing a lot more even after January - I'm thinking of UC Davis, Stanford, and OHSU in particular here, all of which seem to interview into March and even April.
Good luck! It sounds like you've had a lot of success so far (getting interviews for ~half of your schools is great!).