Martin has a reputation for being slow and meticulous. He has expressed some anxiety about the show catching up with him: "I am aware of the TV series moving along behind me like a giant locomotive," he said in an HBO interview, "and I know I need to lay the track more quickly, perhaps, because the locomotive is soon going to be bearing down on me."
But as it turns out, Martin's reputation for slow trackwork is a bit undeserved. Yes, there have been large waits between the release of book in the series - five years between A Storm of Swords and A Feast for Crows, and another six years for A Dance with Dragons. J.K. Rowling finished all seven books of the Harry Potter series in the time it took Martin to write four books of A Song of Ice and Fire. Stephanie Meyer completed all four books of her sparkle-vampire saga in less than the time it took Martin to write A Dance with Dragons.
But these comparisons overlook one key factor - word count. Martin's books are huge. A Game of Thrones clocks in 864 pages in the paperback edition, longer than the first four books of C.S. Lewis' The Chronicles of Narnia combined. J.K. Rowling finished the Harry Potter series in about nine years. Martin only got through four books in the same period, but his overall page count is roughly the same - 4,380 paperback pages for Potter, 4,224 for Ice and Fire through that point.
As the chart below shows, Martin is indeed taking his time - but the pace of his output, page-wise, is on track with Rowling's and higher than just about any of the other fantasy authors listed.
If anything, this chart might underestimate Martin's pace relative to other writers - paperback page count is a good proxy for word count, but not an exact measure of it.
Since his third book, Martin's pace has been consistent - nearly 200 paperback pages' worth per year. Martin has said that he expects the final two books in the series to be about as long as A Dance with Dragons, at 1,500 manuscript pages each, or about 1,150 paperback pages. If his pace continues, we can use the magic of linear interpolation to predict that we should see Winds of Winter come out some time in 2017, with A Dream of Spring hitting in 2023, just in time for Martin's 75th birthday. According to the Social Security Administration, Martin can expect to live an additional nine years after that. Statistically speaking, fears about Martin's premature death are unfounded - we're in the clear, guys!
Of course, if there's any case where "past results do not predict future performance," it would be here. Martin himself is fond of saying his books will be ready when they're ready. All I'm doing here is extending a line. If you want a more sophisticated model to predict when the next Ice and Fire book will drop, 538's Walt Hickey has you covered again. He's factored in things like the sentiment expressed in Martin's blog posts, writing speed of Martin's previous novels, and the performance of the New York Jets to predict that Winds of Winter will come some time between now and 2018.