Two years ago we generally expected Quad Cores to be taken advantage of in only one to two years, and today now that we're there we're still asking the same question. At the moment I will take the wild guess that "Quad Cores Gaming" or "Multi-Cores Gaming" is a gimmick thrown towards us, gamers. On a technical point of view it may be "working", and the advantages measured in extra frames-per-second or better physics effects are barely noticeable, with only a few exceptions where you can say "alright, it does make a tangible difference".
Those exceptions aren't numerous enough to be convincing to us gamers, and those with Quad Cores, from what I've read, think it makes a difference. Is it a placebo effect? Maybe, maybe not. But even if it "works", there's not enough games that do take such an advantage from Quad Cores (the only one I can think of is Unreal Tournament III, because even Crysis don't benefit that much from multi-cores CPU's, and who right in their mind would buy such a CPU just for one game? I hope not a lot of people).
I myself actually wonder why we SHOULD see games taking advantages from Tri/Quad/Multi Cores? Would they be really "better"? What do gamers want exactly, "realistic" physics in a fantasy-based game? Do they want a "more realistic" Flight Simulator XI? No, they generally want better performance, more frames-per-second and generally speaking better graphics, and having a CPU with a thousand Cores won't help much with that. We need better graphics cards right now, and graphics cards with PhysX capabilities are certainly a plus since they can potentially combine all of our wishes for "better games", given that developers actually take the time to look at the possibilities such hardware can provide for gaming.
I still find good Dual-Cores (including those you can over-clock enough) doing the job for gaming. At the moment Quad-Cores are indeed good, but not for gaming alone. Those CPU's aren't "meant for gaming" in the first place. They are made for multi-tasking, along with video and audio editing and encoding, and benchmarking, and folding, and anything you can think of that eats up lots of CPU cycles outside of gaming. They are certainly good, better than Single and Dual Cores for that purpose. But actually why, just why do we gamers need Quad Cores for "better games"? Or even better performance? Shouldn't we ask for GPU's capable of running everything we want at any possible settings? Well yes it'd be expensive, but that would be a better priority in my opinion than having a CPU powerful enough to simulate real-time nuclear explosions and giving us 300 FPS in Crysis at impossible resolutions with 32x AA and God knows what else.
In the end, to answer you directly, no one knows when Tri/Quad/Multi Cores CPU's will really be taken advantage of for gaming. No one can tell, and I'm sure that even Intel and AMD can't tell even internally. Are the developers actually willing to do it for all or even the majority of future games? Do they have the competence to do it "properly"? Do they have enough ideas for their games to actually use all those extra Cores or do they just plan on adding more debris flying all over the place when you fire a rocket launcher and calling that "Quad Core Optimized"? I don't know when it will happen, and no one else can say. It'd just going to happen slowly and over time, that's for sure. It's quite possible that in ten years from now Dual Cores will be something of the far past, and we'll be playing Peggle and Fallout IV and Elder Scrolls V on Deca-Cores CPU's.