In an article on Metal Gear Solid V: The Phantom Pain (containing an interview with Hideo Kojima), website the Guardian mentions early 2015 as the release date for the game, which does seem pretty plausible.
What is interesting is that the release date mentioned in the article was very specific at first: March 17th. But it has since been edited and changed to a more general ‘early 2015’. Here is a screenshot of the date before the change.
It could be that the website had some information that couldn’t be posted yet, and has been removed at the request of Konami. Of course, it could also have been a simple mistake.
In any case, early 2015 seems like a good bet, about one year after the release of the prologue Ground Zeroes. Keep in mind though that Konami hasn’t officially confirmed anything yet.
Update: The Guardian’s editor stated that ‘March 17’ was just a placeholder. They asked Konami for an official release, which is the ‘early 2015’ it was changed to.
Source: The Guardian, PSLS