From Stephen King's official site:
Release Date: June 3rd, 2014
To be released in June 2014, Mr. Mercedes is Stephen's next major publication from Scribner. Stay tuned, more information is coming soon.
During his Chancellor's Speaker Series talk at University of Massachusetts Lowell on December 7, 2012, King indicated that he was writing a crime novel about a retired policeman being taunted by a murderer. With a working title Mr. Mercedes and inspired by a true event about a woman driving her car into a McDonalds restauraunt, it was originally meant to be a short story just a few pages long. In an interview with Parade, published May 26, 2013, King confirmed that the novel was "more or less" completed.
Amazon lists it at 496 pages and pre-orders are available HERE.