Found this on gamefaqs. Lobo was accidentally put up on XBL on April 18, two-and-a-half weeks before his scheduled release. Batgirl going up today probably means that she's intended to release on May 21 -- a new character every two weeks, like MK9. If so, I believe the DLC schedule will follow the order that the characters are listed in the Season Pass coding:
[UnlockedDLC.Characters]
+Unlocked=CHAR_Lobo
+Unlocked=CHAR_Batgirl
+Unlocked=CHAR_Scorpion
+Unlocked=CHAR_Zod
May 7 - Lobo
May 21 - Batgirl
June 4 - Scorpion
June 18 - Zod (4 days after Man of Steel film premiere)
I hope this is the case, because it's a lot sooner than I anticipated.