I've been looking at Punit Soni's Google + account, there is no ETA.
He told me that we have to wait for qualcomm to update the binaries and drivers for the Soc/radios, and then they will update, first unlocked/dev editions, then branded ones.
there is no timeframe, but samsung will update the sgs3 (the US version has the same chipset) in late Q1
as far as ive seen, they will bring kitkat to last years phones, then 2012 phones.
so IMO, i think it will be april/may if you have a phone on contract...
alas, i think its the last update it will receive.
i asked him if they would dump the kernel like they did on the moto x and moto g and they will do so for the xt926 developer edition.
so once KK rolls out, expect better custom ROMs...