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On average, lettuce takes about 30-45 days after you plant the seeds to mature. Make a note on your calendar to begin harvesting after about 30 days has passed.  Indoor lettuce plants grow and mature continually, so you can continue harvesting your plant after you've picked it for the first time. Mature indoor lettuce usually grows to about 4 inches (10 cm) tall. See How to Harvest Romaine Lettuce for specific instructions relating to this type of lettuce. Morning is when your plant's most hydrated and at its strongest. If possible, harvest your plant before the late morning or afternoon to attain a healthier yield. If you can't harvest in the morning, avoid mid-to-late afternoon, which is when your plant's least hydrated. Do not harvest your indoor lettuce plant all at once. As long as you continue taking care of it, you can harvest it for several months. Cut 3-4 of the outer leaves at a time with garden shears or scissors, leaving the rest of the plant to heal and grow back later on. Avoid picking the lettuce's crown or center. Limit yourself to the outer leaves to boost its overall harvesting yield. Depending on the variety, lettuce can last anywhere between 3-10 days in the refrigerator. Check how long your specific variety lasts for and, after placing the lettuce in the fridge, plan to use it by its expiration date. If you don't think you'll use your lettuce in 5-8 days, wait a few days before harvesting your plant. Your plant requires about 2 weeks to heal and grow more leaves before it is ready to harvest again. After your first time harvesting, wait 2 weeks in-between harvesting to keep your plant healthy and able to grow more leaves.  Wait at least 2 weeks before harvesting young plants, which may take time to grow strong after being harvested. Sow additional seeds every 2 weeks to extend your harvest.

Summary:
Begin harvesting your lettuce 30-45 days after planting. Harvest your lettuce in the morning. Cut off the outer leaves. Refrigerate your lettuce for 5-8 days after harvesting. Harvest your lettuce again in about 2 weeks.