If you're intending this for a print, I'm assuming you're going to do some post work in Photoshop or another image editor. I would just break the image up into sections, render in four or more parts and just combine the rendered sections in post if you want to break it up over a few days. My workflow also consists of rendering the image @300dpi but 25 - 50% larger than what my intended print size would be, shrink it down in photoshop add a very small amount of sharpening and have a much sharper and cleaner image than the original. You can also get away with slightly less pixels samples when you shrink down the image, just like a high ISO image will look cleaner at a reduced size, the same happens with rendered images.

Is this a timed work?