Available data and discussion:
As was mentioned earlier, approximately 10 chapters that will be included in TWOW exist and were read by GURM in various conventions or published on his website and other places. The only concrete existing page count - 168 pages submitted in February 2013 - was mentioned by his editor Anne Groell [5]. GURMs own estimates are:
1. Quarter done (April 2013) [6]
2. Hundreds of pages/dozens of chapters, not yet finished. Still months from being finished with good writing progress (January 2016) [7]
Other rumours surfaced through the years (completion by 2016, for example) were disproved by GURM himself. Any information about the book completion status will come from him or his editor. Any other statements should be considered a baseless rumours.
GURM projects the final length of the book at ~1500 manuscript pages (mp's) - his page count, based on the page count of his writing software. The complete manuscript of ADWD was sent to the publishers on April 27th, 2011 (approximately 5 years ago) and was 1510 mp's long. The fourth book in the series was published on October 17th, 2005. Based on the information in [4], more than 500 mp's were written between 2007 and 2010, and the remaining 500 were written in 2010 and the beginning of 2011. This allows us to extrapolate his writing speed.
The "quarter done" statement means ~375 mp's. "Hundreds" could mean anything between 375 and 999, which, based on ADWD progress, could mean anywhere between 1 and 4-5 years to completion. Since this book is highly anticipated due to ongoing TV adaptation, it is safe to assume that its release will coincide with start/end of GOT season. GOT season usually starts in April and ends in mid-late June, which means the possible release dates are April/June 2017 or April/June 2018. The show is expected to run for 8 seasons. Another release date possibility is a winter holiday season, however it is less likely due to benefits of connection to GOT promotion.
Conclusion:
In order for the book to be released in June 2017 (assumed end of GOT season 7), GURM has to have ~1000 mp's by now and write at least as fast as he did in 2010. This is an optimistic, but possible assumption because it is possible he is being pressed by his publishers to progress faster. The more realistic release date projection is therefore April/June 2018, which will mark ~7 years since ADWD came out.