A second lieutenant (grade O-1) is usually promoted to first lieutenant (grade O-2) after 18 months in the Army or 24 months in the Air Force and Marine Corps.The difference between the two ranks is slight, primarily being experience and higher pay. second lieutenant (grade O-1) is usually promoted to first lieutenant (grade O-2) after 18 months in the Army or 24 months in the Air Force and Marine Corps.