Run query every 5 minutes
How about using a PL/Sql block:
DECLARE
    v_sessions NUMBER;
BEGIN
    WHILE true LOOP
        SELECT COUNT (*) INTO v_sessions FROM v$session;
        DBMS_OUTPUT.PUT_LINE ('Sessions: ' || v_sessions);
        DBMS_SESSION.SLEEP (300); -- 5 minutes
    END LOOP;
END;

If it really is 8i, use DBMS_LOCK.SLEEP (300) instead.