I was lucky enough to be invited to a VES event this summer where John Knoll was speaking about Pacific Rim. He went over and "easy" shot (showed us and everyone laught at the thought of that particular shot being easy) which involved Gypsy Danger walking through the ocean. Some intense ocean sims which could not have been "easy" at all. Then he went over the hard shot. He stepped through the Hong Kong battles scen which I believe was about 20 min! After he was talking about the thirtieth different render pass he stopped saying something to the effect of "that was about half of the passes." I would love to see some of those Nuke scripts! Bad picture of the event below.
I posted some VFX breakdown and behind the scene videos below. First video is a 30+ min from FXPHD.