It all depends on the related question, how many seasons will the show run for. Two seasons for Storm of Swords
... which suggests anywhere between two and four for the presumed combination of A Feast for Crows/A Dance with Dragons
. And then anyone's guess how many seasons the last two books are, and this is all assuming the show doesn't get cancelled for whatever reason at some point (or deal with cast disputes - the've already had to write off notable minor characters, after all).