I can’t tell you how long they need to grow in the cups, because the safe outdoor planting time in your region depends on what kind of seeds you are growing and where you live. I’m guessing, however, that most of the seedlings will be fine in the plastic foam cups, as long as you have drainage holes punched in the bottoms and are giving them as much light as possible. A grow-light hung just an inch or two above the seedlings is best. Growing the seedlings at cooler temperatures will slow down their growth, but avoid extreme cold, which could damage the tender plants. The best temperature depends on the type of seedling. For example, snapdragons tolerate cool temperatures, but zinnias need constant warmth to thrive.