I have noticed a refuse to talk length variance. Some AIs seem to talk very fast if they're getting beat down, while others seem to take FOREVER, even if you've taken 3/4 of their cities already they still won't talk...I've had an AI refuse to talk until near-death (well, it had 6 or 7 cities and I took 5, but it was hardly a bad situation for me as I took them in like 7 or 8 turns or something - a time where you'd just kill the target but still amusing that they'd not talk).
Must be coded to vary by AI or situation or somethings.