Some interesting 'facts'
According to Wikipedia...
"To render Avatar, Weta used a 10,000 sq ft (930 m2) server farm making use of 4,000 Hewlett-Packard servers with 35,000 processor cores running Ubuntu Linux and the Grid Engine cluster manager. The render farm occupies the 193rd to 197th spots in the TOP500 list of the world's most powerful supercomputers"
Can't find exact figures, but render time of 5-8hr per frame doesn't seem too absurd.
Of course this render farm was created 5 years ago, so is pretty outdated now. And of course they use CPU's to do all the rendering, not GPU's (AFAIK). Avatar was 4k I think, so rendering at 1080p also gives you quite a speed gain. ie about 20minutes per frame at 1080p.
So a state of the art multi GPU based console running at 1080p would need to be 480times faster that the individual (5 year old) servers used on Avatar, which actually seems vaguely possible.