Benioff and Weiss have confirmed they intend to do 7 seasons of GoT. This means GRRM has 3 years left to get two (or more) books done. Based on GRRM's past this appears unlikely. In fact since GRRM has given the ending to Benioff/Weiss there is a chance that show watchers could end up spoiling the ending for book readers.#ironic

"When the snows fall and the white winds blow, the lone wolf dies but the pack survives."
Winter is Coming

Now this is the Law of the Jungle—as old and as true as the sky; And the wolf that shall keep it may prosper, but the wolf that shall break it must die.
As the creeper that girdles the tree-trunk, the Law runneth forward and back; For the strength of the Pack is the Wolf, and the strength of the Wolf is the Pack.