A fan mock-up of the potential cover to the book.
To clarify, Martin himself has not offered an update on the book's progress since April 2013 when, on the carpet for a Game of Thrones TV event, he confirmed that about a quarter of the book was complete (very roughly 375 manuscript pages, assuming the book will be as long as A Dance with Dragons) with additional material in drafts and partial chapters also done. Subsequently, he offered upbeat assessments that he was writing fast to stay ahead of the TV show (which will begin introducing plot elements and even characters from Books 4 and 5 in the fourth season, due to air from April to June this year) but not specific page counts.
To focus on The Winds of Winter, Martin eliminated his outstanding side-projects in the first half of 2013, completing editing work on several anthologies (some of them now out) and concluding his contribution to The World of Ice and Fire companion volume. Several other works that were judged to be too time-consuming have either been cancelled completely (a short story for a Poul Anderson tribute anthology) or put on the back-burner until either the completion of The Winds of Winter (most notably the next Dunk 'n' Egg short story) or the completion of the whole series (a second and more in-depth companion volume, tentatively dubbed the 'GRRMarillion'). The result is that Martin has certainly had time to make inroads on The Winds of Winter.
Last month, a website randomly said that Martin had over a thousand manuscript pages completed for the book. Martin later confirmed that he had said no such thing, whilst not offering an update on his estimated page count for the book. It may be that we will get an update on the book during the inevitable press clamour for Season 4, but also possibly not: after providing updates on A Dance with Dragons that didn't seem to help the situation, Martin has indicated that he will be providing far less frequent updates on The Winds of Winter, if any at all.
However, it should be noted that George R.R. Martin has also said he wants to stay ahead of the HBO TV series if at all possible, and this will only remain a realistic goal if The Winds of Winter is released in 2015 with A Dream of Spring to follow, at the absolute latest, in 2018.
As always, believe nothing (not even this post!) until it comes from GRRM directly.