Maybe by the time he finishes The Winds of Winter HBO will be ready for the 7th season (I'm saying 7th instead of 6th because GRRM mentioned on his blog that the 3rd season of GoT will probably only cover the beginning of the 3rd book).
I tried my hardest to read A Feast for Crows. I tried, but I just couldn't do it. It's like he used all of his creative talent on all of his other books (currently written), then had a 2-year-old write it.
He doesn't give out definite time lines because he really doesn't want to hear the complaining. According to his website there is a tentative 3-yr date for the finishing of his 6th book. I'm not holding my breath and will wait until it comes out. There are more than enough books to read until it does come out.
I think he should take all the time in the world if that's what he needs. I'd rather wait for a long time and have a satisfyingly good book to read, rather than not wait at all and just be given trash.
It takes so long, because GRRM vision of the story is so detailed and long...... and the fact that he tell so much of the story....
SOFAI was meant to be a trilogy..... it is currently looking to be 7 books.... but who know.
The next book should be released quicker than the 6 years it took for Dances as he has a good amount already written (ie, chapters that were planned for Dances, but pushed to the next book due to Dances getting too long)
I love the story, so I'm willing to wait so that it can be told in the best way.
If you're really needing another fix, there are two short ASOIAF novellas in "Legends" and "Legends II", edited by Robert Silverberg. And yeah, I know quality takes time and I'd rather he write the book his way, but still.... it's like waiting for Christmas for years!