It'll probably be announced at Blizzcon this year in November and even if the beta starts that day you're looking at at least 6 months of beta until the game is released. So the earliest it could be is May of 2014. And if SoO is released in September, 6 months after the release of ToT in March, you're looking at 9 months between SoO and the next expac, which is about how long Dragon Soul was around.