I'd like to say "X Y Z" date, but it doesn't work that way.
According to WoWWiki, "Blizzard has stated it plans to release one expansion per year for World of Warcraft. Yet development of new expansions does seem to take significantly longer." World of Warcraft was released in 2004, and as of 2009, there have only been 2 official "expansions", even though there are several content patches each year, adding new dungeons, zones, quests, mounts, zones, items, pets, and more.
To get an idea of when content has been released in the past, you may want to look at the listing of previous Patches.
Expansion ideas (created by users) you might also find interesting.