How long should I leave the seedlings in the red s...

A general rule of thumb is to transplant when the leaves start to reach the edge of the cup. By then they should be strong enough to handle the transplant but not root bound. Happy growing

You should transplant when you see signs of the plant being root bound i.e. check the drainage holes at the bottom of the container—a plant should have a healthy and visibly white root system. If roots are growing out of the holes, it’s time to transplant. Any discoloration or darkening may indicate the plant has become rootbound and a transplant should take place immediately. The symptoms of a rootbound plant include: Flimsy new growth Stunted flower production Stem discoloration (reddening) Nutrient sensitivity Nutrient deficiency I would recommend an intermediary transplant to a 3 or 5 gallon container before graduating to 7 and 10. This will allow the root system to develop more. It is easy to overwater a small plant in a large container which can lead to issues such root rot. Best of luck!