My feeling tells me that your processing time all depends on how long it takes to do your background checks. You may get called for test and interview at any time while they are still doing these checks and then have to wait ages for your Oath. Or maybe they'll call you for the test after the checks are done and the oath comes quickly after that. It's not a first come first serve process at all. Every case is different and the time it takes to process is different.
Thanks for you reply @bucajack. I called them early November and they said my background checks were done and is all clear. If I may, what form of correspondence do they prefer? E-mail or snail mail? I'm in no great rush, just curious on the process. Thanks