Depends on that person, either he/she uses CoO ability and immediately acts on it or uses it but doesn't acts on it. It seems auto dodge for most people as we've seen so far in both anime & manga versions, but for me, anyone that had such haki won't be too stupid to not respond to it. Like, you got an early warning over something risky which would surely be happened to you, i didn't think you wanted to stand still, received it and went died. At the very least, you would want to dodge, despite how fast/slow you could do it. Also, it was akin to our body-reflex as well, normally transpires even to a non-haki user.
For example, Brook. He didn't have haki, right? But remember during Punk Hazard, as he was making some snowmen while waiting for Luffy & co. to arrive near Sunny, his hand spontaneously moved away from Kin'emon (torso) slice attack without Brook himself realised it at once. He appeared shocked though soon after. Well, it was...things like that.