Later this month.
Here a synopsis for the DLC story:
Set prior to the events of Batman: Arkham Asylum, Batgirl: A Matter of Family will allow players to take on the role of Batgirl for the first time in the Batman: Arkham franchise. This exciting story add-on features an all-new location with multiple missions, side quests and secrets; a new hacking feature to progress through the world and solve puzzles; and Dual Play functionality with Robin for dynamic take-down manoeuvres.
Here’s a head up: The DLC will be developed by WB Montreal, the ones in charge of Batman: Arkham Origins, and not Rocksteady studios. Yeah, just thought you might want to know…