PD's been working on GT6 since 2010... I'm not sure EXACLY when development started, but Kaz stated in an interview that GT6 was already under development and this was before right GT5 was released. I wonder just how extensive this "development" was. If 100% of the workforce of PD had been working on GT5 and it's DLC, we'd probably be getting monthly updates and DLC just as Forza 4 does, just with about the same amount of content per update/DLC each time. If PD had put 100% of their workforce on GT6, then it possibly could've been released next year or so. I'm expecting a possible release of between holidays of 2013 - 2015. GT6's development should be shorter than GT5's, now that the new engine is developed, at least partially. GT6 will most likely be just edits (and maybe big re-builds) to the physics engine, but nothing compared to starting from scratch. I'm expecting 1000+ cars, hopefully with better variety, (basically, Forza 4's car list, just doubled) and a good amount of new tracks... GT5 is a bit low on those, and we know we'll be getting Bathurst, and we STILL don't know whats going on with those leaked track signs.
I hope we get at least one more DLC pack after GT Academy, and hopefully a "Spec III" update.