Stef,

Maybe this query might work for you.

WITH CTE

AS (SELECT J0.*,

           (SELECT COUNT(*)

            FROM TEST_TABLE S1

            -- LEFT OUTER JOIN TEST_TABLE S2 ON S1.ITEMNR = S2.ITEMNR AND S1.DATEUSED = S2.DATEUSED - 1

            WHERE S1.DATEUSED >= J0.START_DATE

                  AND S1.ITEMNR = J0.ITEMNR

                  AND S1.DATEUSED < J0.END_DATE) + 1 DAYS

    FROM (SELECT DISTINCT J1.ITEMNR,

                          J1.DATEUSED START_DATE,

                          (SELECT FIRST 1 S1.DATEUSED

                           FROM TEST_TABLE S1

                           LEFT OUTER JOIN TEST_TABLE S2 ON S1.ITEMNR = S2.ITEMNR AND S1.DATEUSED = S2.DATEUSED - 1

                           WHERE S2.ITEMNR IS NULL

                                 AND S1.DATEUSED > J1.DATEUSED

                                 AND S1.ITEMNR = J1.ITEMNR

                           ORDER BY 1) END_DATE

          FROM TEST_TABLE J1

          LEFT OUTER JOIN TEST_TABLE J2 ON J1.ITEMNR = J2.ITEMNR AND J1.DATEUSED = J2.DATEUSED + 1

          WHERE J2.ITEMNR IS NULL

          ORDER BY 1, 2) J0),

CTE_SUM

AS (SELECT ITEMNR,

           MAX(DAYS) DAYS

    FROM CTE

    GROUP BY ITEMNR)

SELECT CTE.*

FROM CTE

INNER JOIN CTE_SUM ON CTE.ITEMNR = CTE_SUM.ITEMNR AND CTE.DAYS = CTE_SUM.DAYS   

I hope this helps,

Edward Mendez