Transplanting should be done when the roots are hitting bottom. Stunting or delay of growth can only occur if a container becomes root bound. Most trees will never recover their real potential, but weed will. Most grasses will as well. It may delay it, but not stunt it. So, better to be a bit late then too early.
If you feel that the plant has 'taken" the pot (heh, pot), then gently turn it upside down (about 60 degrees downward will suffice) and slide it out and have a look at it. If you start to see an accumulation of roots near or at the bottom, then go ahead and transplant. If you don't just gently right the plant and no harm, no foul. I find it always best to maintain a rhythm when growing any plant.