It is normally about a week or 2 before the judges auditions start, however, if all 4 judges return we normally know by now, as if I remember correctly series 5 - series 6 we found out the panel in end of March/April time as all 4 judges returned.
However series 7-8 we found out very late on as a panel overhaul took place.
So, I think it is pretty obvious a judge will be leaving at the least, so we have a bit of a wait yet, the odd judge may be announced who is returning but we have a wait for all 4 to be confirmed