Dairies on the convert to raid podcast let it slip that he was having fun playing the demon hunter in the alpha.
@1:07:10 https://youtu.be/9SlYXWWQ4G4?t=4030
Warlords of Draenor entered alpha testing in April 2014 and the beta started in June 2014. This video was released on August 12th and today is october 27th, which means the Legion Alpha has been going on for roughly the same duration as Warlords did.
Chances are Beta might actually start around Blizzcon.