wait until the plant is nearly rootbound, slowly squeeze the cup as you spin it in your hand to loosen the soil a bit, then put your hand over the top of the soil with the stem between your fingers and turn it upside down. Slowly let the rootball rootball slide out and check to see if the roots are swirled around the bottom. if they are, transplant. If not wait till they are. This is easier to do when the soil is moist so it holds together.