Calculate the average length of employment for employee working in the Research and Development deparment.
average length of employment = AVG(subtract(2022, year(HireDate)))
SELECT AVG(STRFTIME('%Y', CURRENT_TIMESTAMP) - STRFTIME('%Y', T1.HireDate)) FROM Employee AS T1 INNER JOIN EmployeeDepartmentHistory AS T2 ON T1.BusinessEntityID = T2.BusinessEntityID INNER JOIN Department AS T3 ON T2.DepartmentID = T3.DepartmentID WHERE T3.Name = 'Research and Development'