How many years were there after Don Waddell retired and became a coach in NHL?
after retired and became a coach refers to max(subtract(year, lastNHL))
SELECT MAX(T2.year) - MIN(T2.year) FROM Master AS T1 INNER JOIN Coaches AS T2 ON T1.coachID = T2.coachID WHERE T1.firstName = 'Don' AND T1.lastName = 'Waddell'