They'll have to change it from the books if they outpace them at this rate. Won't be any more series to do otherwise.
It is known.
I reckon that if GRRM can wrap up the book series in 7 books and he doesn't suffer any delays as for book 5, it might be
just possible to get both series to conclude simultaneously. Bear with me on this...
HBO just renewed the show for seasons 5-6, and the show-runners (who have been told by GRRM how the story ends) say they can wrap it all in 8. This season is finishing off book 3 and getting started on books 4 and 5, weaved together in chronological order.
The big delay in getting book 5 out was GRRM doing a rewrite when his initial idea of an in-story five year jump required too much in the way of exposition and flashbacks, plus trying to work out how to get the Essos and Westeros storylines to finally converge ("the Mereenese Knot"). A certain amount of the delay in writing book 6 has been GRRM getting distracted by side projects, but he had a small luxury of several chapters from the book 5 draft held back.
Without any more displacement activity, there's a hope (in the fandom at least) that book 6 comes out next year, and
if GRRM can wrap up ASOIAF in 7 books, the last one might be out in say, 2018. Subject to him not doing a Robert Jordan or Frank Herbert and dying before the series is concluded...
So, an optimistic projection might be:
Season 4 - filmed 2013, on TV now, depicting books 3/4 (+5?)
Season 5 - filming 2014, on TV 2015, depicting books 4 and 5 - assume book 6 published early(ish) in 2015
Season 6 - filming 2015, on TV 2016, depicting books 5 and 6
Season 7 - filming 2016, on TV 2017, depicting books 6 and the show's makers interpretation of what GRRM has told them
Season 8 - filming 2017, on TV 2018, depicting the show's makers interpretation of what GRRM has told them - book 8 published in time for the start of season 8
Best taken well salted, but it's no more implausible than mrcharly's post above mine - though he's not far off wrt Cersei, Tyrion and
Theon Reek.