Well you want the seedlings to be strong enough to survive the transition from indoors to outdoors. One of the most important things I have found is to have a fan oscillating to imitate the wind. This helps them to strengthen the walls of their stems creating a healthier plant. I do tomato and peppers 8 weeks before planting. Flowers sometimes take a bit longer like coleus and impatiens, salvia and zinnia. Six weeks out I start lettuce and greens in plug trays and then transplant. It all depends on what you are growing. Plenty of light and air are very impt. Have fun and keep me posted.