Sohn first competed in the 1,500 and 5,000 m, but turned to longer distances after winning an eight-mile race in October 1933. Between 1933 and 1936, he ran 12 marathons; he finished in the top three on all occasions and won nine. On November 3, 1935 in Tokyo, Japan, Sohn set a world record in the marathon with a time of 2:26:42. According to the International Association of Athletics Federations, this record remained unbroken until Sohn's own trainee, Suh Yun-Bok, won the 1947 Boston Marathon.
Question: How many years did it take Sohn to finish 12 marathons?
Answer: 3