There's no announced date for the release, but a beta is expected sometime later this year (2015).
http://www.pcmag.com/article2/0,2817,2489196,00.asp
UPDATE: The release date for Legion is now "Summer 2016" but the beta is still expected sometime later this year. Signups are available at Battle.net.
UPDATE #2: Stuff about the new class of demon hunter and more.
http://www.wowhead.com/legion#new-class-demon-hunter
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