Considering the fact the games industry is dominated by consoles, it is don't argue, just how long will it be before PCs bought this year (SB system with 6950/560ti+ gpus etc) become incapable of playing AAA titles at acceptable fps and resolutions? Let's say BF3 at 1080p and high, assuming high is not max, settings?

We can assume that the next generation of consoles is at least 2 to 3 years away by the current trends. So I really can't imagine a higher end machine even struggling with AAA titles for until the first year of a new batch of consoles. Strictly speaking future titles prior to these consoles will test machines, just because of much higher resolutions and the PC ports usually being higher end*. But I really doubt we'll see machines based around SB having any trouble, only those on the bleeding edge will feel the explicit need to upgrade.

*PC ports are sometimes higher end or have additional options, i.e. Dragon Age 2 released a higher resolution pack and Crysis 2 the Dx11 mode.