CREATE TABLE table_43728 ("Stage" FLOAT, "Start" TEXT, "Arrival" TEXT, "Duration" TEXT, "Difficulty" TEXT)

How long does the one that stars with son bou of middle difficulty, and on a stage number larger than 13 last?
SELECT "Duration" FROM table_43728 WHERE "Stage" > '13' AND "Difficulty" = 'middle' AND "Start" = 'son bou'
SELECT "Duration" FROM "table_43728" WHERE "Difficulty" = 'middle' AND "Stage" > '13' AND "Start" = 'son bou'