I'm at the fantastic stage of being able to see both Hawks and Flamehawks - not a problem, really. Just wondering how long the overlap lasts? I've heard it's only for rank 350, which'd make sense, but just curious as to if anyone knows for sure, cause I really don't need so many crimson obelisks.

If your still getting Hawks, take advantage of it while you can so your can get the Seeing Red medal. If you don't care about the medal then don't worry about it, but you do need 500 non-buffed Crimson kills, so kill any and all hawks while you can to chips away at that particular medal.
As for the overlap, I believe its at rank 400, but I don't remember. I know at rank 400 I never say a Hawk again, but I could be mistaken and the overlap stops at like rank 375, 380, etc I'm not sure.

I'm at rank 394, I haven't noticed them go away yet, I was assuming they went at rank 400.