When is Game of Thrones book six scheduled to be released?

When it's done (and by the way: the series is called "A Song of Ice and Fire," "Game of Thrones" is just the first book... and the title of the TV series). Assuming it's being written at the same pace that the fourth and fifth books were, that would be probably 2016 or 2017. There are indications it may be sooner than that (Martin had originally planned it for Summer 2012, then October 2013), but ultimately it's impossible for anyone other than Martin himself and perhaps his editor to know how near it is to being ready, and even they don't know exactly when it will be ready.