The current expansion is barely one year old (released September 2012), and while I still believe their ultimate goal is to be able to shorten the period between expansion packs to 18 months, I don't think they're going to be able to do that this time. If I had to guess, I'm going to say mid-to-late summer 2014, with the standard delay to early fall to August-September (i.e. I'm a little more optimistic than lupi ).
Hard to believe that panda land is only 1 year old, seems like forever! Probably seems longer because most of us got beta access (something I don't want this go around, spoiled some of the early fun )