When you have learned every aerobatic maneuver that trainers can do and it's been boring for awhile. That said, all those maneuvers are easier on a sport plane, so in a way sticking with a trainer holds you back from learning. If you already have a sport plane, I'd say move on to it as soon as you stop bouncing your landings and generally beating up your trainer. If you don't already have one, fly your trainer until you're bored to tears with it.</p>