When it comes to transplanting your seedlings to the garden or an outdoor container from indoor pots, there isn’t a definitive answer on the best timing.
It’s not how tall the plant is that determines its readiness for transplanting. The general rule of thumb is that your seedling should have four or more complete leaves, which the plant can use to sustain itself. The leaves that were the first to appear and have the shape of the seed itself are not considered complete leaves, nor are leaves that are curled or still holding onto the seed coat.