Question: For all the employees that have left the Engineering Department, what is the average time of their stay?
SQL: SELECT CAST(SUM(365 * (STRFTIME('%Y', T1.EndDate) - STRFTIME('%Y', T1.StartDate)) + 30 * (STRFTIME('%m', T1.EndDate) - STRFTIME('%m', T1.StartDate)) + STRFTIME('%d', T1.EndDate) - STRFTIME('%d', T1.StartDate)) AS REAL) / COUNT(T1.BusinessEntityID) FROM EmployeeDepartmentHistory AS T1 INNER JOIN Department AS T2 ON T1.DepartmentID = T2.DepartmentID WHERE T2.Name = 'Engineering' AND T1.EndDate IS NOT NULL