What is the longest time for a student from the navy department to be absent from school?
navy department refers to organ = 'navy'; longest time to be absent from school refers to MAX(month)
SELECT T1.month FROM longest_absense_from_school AS T1 INNER JOIN enlist AS T2 ON T1.`name` = T2.`name` WHERE T2.organ = 'navy' ORDER BY T1.`month` DESC LIMIT 1