Official answer to this was (at the assesment day): 12 months after date of rejection no matter what stage you were at. However, she did mention that becuase it took time for the initial application to be processed, you can apply a couple of months before this (so between 10-12 months). Reason being, if you wait the full 12 months then adding the time BA take for the initial application assessment, it could be anything up to 15 months since your last invite/decline - so they give you a bit of a headstart that way, and they should!