Leaf lettuce can be harvested continually throughout the growing season until the plant starts to bolt or otherwise expires, as long as you harvest only the outer leaves each time and leave enough inner leaves, and the inner growing center, for the plant to continually make new leaves and make food for itself. Those inner leaves will then mature and become the “outer” leaves, and you can harvest those. (The cut stem base of the harvested leaf will not re-grow into a new leaf.) Heading lettuce, however, is usually just harvested once after the head matures.
Leaf lettuce can be harvested continually throughout the growing season until the plant starts to bolt or otherwise expires, as long as you harvest only the outer leaves each time and leave enough inner leaves, and the inner growing center, for the plant to continually make new leaves and make food for itself. Those inner leaves will then mature and become the “outer” leaves, and you can harvest those. (The cut stem base of the harvested leaf will not re-grow into a new leaf.) Heading lettuce, however, is usually just harvested once after the head matures.