Depends on the school!
If I remember correctly, the earliest interviews take place sometime in Oct-Nov. So, I believe the earliest rejections will be sometime in Dec.
Some schools won't notify the applicants of their rejection, until they're done with the whole admission process, in late April or so!
Why would you care though?!
Hehehe... you can definitely get rejections earlier than December - I applied to Harvard for the hell of it, and got my rejection in like November, hehe.