Such a set could launch in late 2013 or early 2014, although a potential launch timeframe has been a moving target for Apple observers over the past several years as the company has apparently yet to make its TV work into a formal project.
It will launch when component prices have fallen to the point were Apple can make it and sell it for a reasonable price and still make their usual margin. Right now an Apple Ultra HD TV would be so expensive it would have limited appeal.