It's never really too early to transplant. When someone sprouts seeds using the paper towel method, it's technically transplanted when put in their new home. But, the less root system developed the more risk that something could go wrong. A general way to think about it is that the root system is around the same size as the plant is. I basically do the same thing as ANC. When the leaves reach the sides of their container, I transplant. That allows for a good root system, while not becoming root bound. But it's also ok to wait a little longer.