Where is Grand Admiral Thrawn Event?

Palpatine and Yoda have been around a long time and went into an official every other month rotation. R2 came back once specifically because he was needed for the CLS event. There is no indication of how often he will return outside of that. It hasn't been that long since the first Thrawn event - Palpatine didn't return for longer after his first event.

My guess is around October 16th when rebels returns

He's over there.