EDIT: Ah, user end balances are supposedly correct, that would be evidence supporting the word of the hackers.
My last trade was on 2014-01-23, and the balance in the leaked data is correct for what my BTC balance was at that point. So, apparently it happened on or after that date. If enough people post after checking their accounts in the leaked data, we can determine the earliest date the leak could have occurred by consensus. At least for the final user balance dump.