NOT SPOILERS even though I am mentioning books here, so don't worry people
This is what I have envisioned for the series based on various speculation about HBO's production schedule desires and the length of the books plus their structure
Season 1 = Book 1
Season 2 = Book 2
Season 3 = Book 3 first half
Season 4 = Book 3 second half, plus elements of books 4 and 5 to setup next season
Season 5 = Combined books 4 and 5
Season 6 = Combined books 4 and 5 plus starting book 6
Season 7 = Ending book 6 plus starting book 7
Season 8 = Ending book 7 and the series
This means the final book would start airing in 2017 and finish in 2018. If GRRM puts out book 6 in late 2014/early 2015, this only gives him 2-3 years to finish the final book and publish it. Meanwhile, HBO will already be filming the final season before the book is released. Things don't look too promising for GRRM but I have a feeling after this next book is released he is really going to hammer down and focus on finishing the final book 7
All speculation of course. But it just illustrates the very real possibility that the show will end first and spoil the books for all the book readers