Whatever speed you choose when you what to enter the event horizon. If you stay in the horizon you will at all times past present and future. But first you need to be able to live through the Massive gravity of the wormhole.
There is actually a lot of research going on relating to this subject. The consensus is that there is a possibility, but not back any farther then when the machine was turned on. Don't ask me for evidence, because it was in a documentary I watched.