Once the leaves are long and slightly overlapping, they form a head that is six to eight inches long. Romaine lettuce takes about 70 days to mature but can be picked sooner if you prefer younger leaves. With some pruners, cut the head close to the stem about an inch away from the ground. This can also be harvested when the leaves are younger, and you can take the whole head of lettuce for tender leaves. If the head is cut an inch from the soil, the lettuce may start to grow again for a second harvest.