Loose-headed leaf lettuce when the leaves are large enough to use—just 2 to 3 inches long.
Bibb and Butterhead types of lettuce—with large ruffly outer leaves surrounding a soft, folded heart—when the leaves begin to cup inward to form a loose head or wait until they form a rosette at full size—6 to 8 inches across.
Romaine lettuce also called Cos lettuce, when the leaves have elongated, formed midribs, and overlapped to form a fairly tight head–about 6 to 8 inches tall.
Harvest Crisphead lettuce—like Iceberg with a solid, tight, spherical head–when heads are medium-sized (6 to 8 inches), round, and firm.
For more information on growing Lettuce, check out this great guide from Harvest To Table: