CREATE TABLE table_204_598 (id DECIMAL, "name" TEXT, "state of residence" TEXT, "took office" TEXT, "left office" TEXT, "president served under" TEXT)

how many years did david plouffe serve ?
SELECT "left office" - "took office" FROM table_204_598 WHERE "name" = 'david plouffe'
SELECT "left office" - "took office" FROM "table_204_598" WHERE "name" = 'david plouffe'