2 KDR is the minimum for being considered decent imo. if you're trying and do the occasional mess-around class, then 2 KDR should be achievable fairly easily. 2.5-3+ takes a lot more consistent effort and trying hard to maintain, especially in objective game modes, assuming you're primarily going for the win. the game is all about the win for me and i personally try to maintain a 3-4+ WL in correlation with my KDR. when you see someone with a 3+KDR and a .9 WL, then it's not impressive, even if you play solo. you should be able to maintain a 2 WL as well if you only play solo, besides major team game modes like hardpoint, etc.
that being said, all CoDs aren't equal and all have a different learning curve.
MW2 - 1.7X KDR (this was my first CoD and i've been raising this from .8 KDR since i've owned the game.)
BO1 - 1.5 (i never liked this game, so i only have 4 days played. i've played it recently and have been raising it slowly.)
MW3 - 2.08 KDR
BO2 - 2.35 KDR
Ghosts - 2.6X KDR