I'd say we'll be able to do Gollum in real time on a PC in 8 to 10 years, and another 5 to 10 years beyond that we'll see something that could legitimately be described as "photorealistic".
I'm inclined to agree with previous posters that problems with animation are actually more significant than problems with rendering.
In one respect, though, games have an advantage over movies: in the game, everything is CGI. You therefore completely circumvent the problem of lighting on a CGI image not matching the live-action background - here, there isn't any live-action background, so the lighting will be entirely consistent.
My biggest problem with Gollum is actually not the lighting but the lack of mass. There are some scenes where they actually had Andy Serkis in the scene when it was filmed, digitally painted him out, and put Gollum in instead; that sometimes works very well indeed, for example in the scene where Gollum pulls Frodo out of the pool in the Dead Marshes - there's real substance to Gollum there. But in other scenes there's no physical interaction between Gollum and his environment: objects that he grabs hold of remain absolutely motionless, and his feet don't disturb a single leaf or twig on the ground.
Personally I think there's one thing that would add enormously to immersiveness in games, and that's a return to Virtual Reality technology. I don't care very much about stereoscopic 3D, and most post-VR technologies (e.g. shutter glasses) have (wrongly) focused on that aspect of it. The thing that would hugely add to immersion is to have a system with head-tracking, where both the graphics and the sound are updated dynamically according to the player's head movements.
It was this that never worked properly with the original VR systems (because of low frame-rate and lag - this was what led to motion sickness) but it's this that makes the difference between viewing an obviously flat image on a monitor and actually looking out of a window. No matter how realistic a scene is (even if it's stereoscopic), as soon as you move your head even the tiniest bit, there's no parallax, and that completely destroys the 3D illusion.