Programmed refresh of views materialized in Oracle

I'm trying to refresh a view with REFRESH FORCE ON DEMAND to perform a refresh in the next time window: Every day from 08:00 to 23:59, with a period of 4 hours between refreshments.

I'm using

REFRESH FORCE ON DEMAND START WITH sysdate+0 
NEXT CASE
    WHEN  TO_CHAR(SYSDATE,'HH24') IN ('24','01','02','03','04','05','06','07') THEN
    TRUNC(SYSDATE,'DD') + 08/24
    ELSE
    SYSDATE + 4/24
    END

But I can not get it to work as I want. Does anyone have any idea where I may be failing? Thanks in advance!