I applied to seven PhD programs, and I've received two interview invites so far. I haven't heard back from the other five yet. I received one over this past weekend, and one on Dec. 21. I think I should hear back from all by the end of January, but you can check the results search to see if anyone else from your program has heard back yet. You may not see any there if you applied to very small programs though. I know at the institution where I work now, the committee has been meeting each week and has finished the domestic apps, but won't send invites until next week when they finish the international applications. It just varies by program. One program I applied to didn't even have a deadline until Jan 15, so I have no idea when to expect to hear from them.