List the longest duration of absense for a student enlisted in the fire department.
longest duration of absence refers to MAX(month); department refers to organ; organ = 'fire_department';
SELECT T1.month FROM longest_absense_from_school AS T1 INNER JOIN enlist AS T2 ON T1.name = T2.name WHERE T2.organ = 'fire_department' ORDER BY T1.month DESC LIMIT 1