I'm fairly confident we'll get The Winds of Winter before the show overtakes the books.
I think there's a very good chance the show will get into at least some material, if not a whole lot of it, from Winds
before it's published. It looks like they're going to massively condense and speed through Feast
. I think Dany will be in Westeros by season 5.
It's probably just the last book that might get passed over. If that does happen, I'll wait to read the book before I watch the end of the show.
That could be a long wait. If GRRM takes five or six years to get each of the sixth and seventh books out that would lead to the final book being published sometime between 2021 and 2023, while the show will end in 2018 if it runs for eight seasons.