The total render time was about 17 hours the last time I did a full frame at 3840x2400, which I'd consider working size for this image because of all the detail. I don't think that's bad at all. Since the image is tiled I can do sections overnight and keep the machine free during the day. So a couple of overnight sessions and it's done.
Right now I'm rendering small patches of the image at that resolution to find the optimization sweet spot. So custom sampling settings, etc., are animated over a series of small crops. Those run overnight too. It's a gradual process but it lets me move on to other projects during the day.