If we assume that 1 book = 1 season of episodes, then there is enough for 4 seasons right now, with the 5th book due out in July. The 3rd book might be good for 2 TV seasons though, it's a much longer book.
HowEVUH, Martin is, charitably, a slow writer. 4 books in 15 years. Might have a few Sopranos-esque delays in between seasons.