We don’t have a fix date but Nvidia has talked to some of its partners assuring them that they will have something new and hot to see as of May 2013. Computex 2013 starts in late May but Nvidia might launch its new graphics card generation a bit earlier.
They do want to address AMD’s aggressive bundle push and try to offer a more competitive product. Bear in mind that desktop sales still play an important role in Nvidia's financial statement. We don’t know much about the performance expectation but we do know that it is faster than what is out today.
At 28nm you cannot expect miracles, but next year Nvidia does have plans to move to 20nm graphics and put even more transistors on their graphics dies. Nvidia also wants to put pressure on new console sales, as now more than ever Nvidia wants to tell the world that PC gaming is the future.