Strictly correct, I suppose, as many pax can't reserve any seat until T-24. But for the OP the useful answer is "3 days", when the seat does become available to all those entitled to advance seat assignments. Note it is not exactly, but only roughly, 72 hours.

At T-3d, when babies etc. are allocated seats as explained on ba.com, 64A will show as available in MMB if it hasn't been allocated to a baby-carrier.
However, if you try to move to it using MMB, the little man will bounce straight back to where you were before. If you want to get the seat at that point, you will have to phone BA as it gives them a chance to warn you that you may be booted out of it by a baby.

I have done this, as have countless others on this board, and it works. However, most people don't know about it, so unlike the other star seats this seat is often still available at T-24 which is the first time you can reserve it via MMB.