What was the average time in milliseconds Lewis Hamilton spent at a pit stop during Formula_1 races?
average time in milliseconds spent at pit stop refers to AVG(milliseconds)
SELECT AVG(milliseconds) FROM pitStops AS T1 INNER JOIN drivers AS T2 on T1.driverId = T2.driverId WHERE T2.forename = 'Lewis' AND T2.surname = 'Hamilton'