Depends on the width of the tank, front to back. If its a 40L, and only 12" front to back, then you'll need to upgrade when he gets to about the 10-11" mark, at the latest. If its a 40 beeder, and 18" front to back (or whatever it is), then you've got a little longer. You could maybe go till 13-14". But it would be better to go larger before the 12" mark, as it will appreciate the larger water volume, and extra length and width, a lot.