http://us.battle.net/d3/en/forum/topic/8415171902#9
My response to someone asking if we’d see the first expansion in two months time…. tl;dr – no way.
Highly unlikely that we will get any official word in the first half of 2013. The meta-game still has to have PvP properly implemented before they even officially acknowledge a release date for the first expansion. Furthermore, Rob Pardo has already stated the expansion will have a public beta (one can refer to Heart of the Swarm to get an idea of the time frames as well as methods employed – http://us.battle.net/sc2/en/blog/7157306/Heart_of_the_Swarm_Beta_Now_Live_-9_4_2012).
Diablo III was originally slated to release towards the end of 2011 but was moved back to its May 2012 date. Closed beta began in September 2011 with an open beta weekend stress test in April 2012 prior to the game going live (leading to a longer beta but with more participants).
Basically, their recent closed public betas seem to last around 6 months.
If anything, any official announcement will likely come at BlizzCon in November. If you go by this recent trend, this would put an expansion #1 release somewhere in the first half of 2014.
