He has been taking 10-15 wickets every 2 tests. At that rate he would be looking at the 3 at the earliest and the 4th at the latest. With the tests being played later in the summer which means the wickets will be dried out sooner I think the 3rd test he will be making his 600th wicket in the second innings. Maybe he can claim his 600th at the same time Australia wrap up the series.