6 Years is a very decent time frame. A one year head start goes a long way, that is the reason MS pushed the 360 into the market as soon as it could. Moreover, MS and SONY compete on equal grounds, target the same market (more conventional gamers are still their core target), that means that they have to compete on the quality of the established variables (creating the awe effect through improvement in the existing formula). If the next XBox comes out in fall '12, 360 will go down to $129-49 range and unlike the original Xbox the 360 is a very capable platform with a perfectly competitive library. That would mean SONY would have to compete at that price level with the PS3 and introduce the next gen console within a year so that the competition does not have a significant head start in sales.
The current gen is already showing its age (compared to PC) and I am sure that when the next generation of GPUs hit the market (especially the next architecture on 28nm). The graphics game is going to move into the next generation very quickly.
As far as "getting to know the new hardware goes", 30% utilization of next gen hardware looks better that 100% of current gen. Making stuff look better is just so much easier.
So I would expect a new Xbox by fall 2012 and PS4 fall 2013 and GT6 as a launch title - or releasing 1 year within the launch bracket. (With a decent expansion pack - or smaller packs - of premium cars and maybe some tracks before 2013).
All I am try to say is that the masses decide what is going to happen next; the players in the industry just have to submit to their will or risk getting left behind.