SQL due date query issue, excluding weekend

Hi,

I'm using workflows and I need to set due date as 56 business hours from the date request has been created ( 7 business days).

Initially I used. 

to_char(cr.request_date+7, 'YYYY-MM-DD HH24:MI:SS') as due_date

which only sets due date after 7 days, which doesn't exclude weekends as it counts weekend as a day too.

Later. I've used this.

select request_date + (select max(rnum) seven_business_days_from_now from
(select rownum rnum from all_objects) where rownum <=7
and to_char(to_date('01/17/2004','mm/dd/yyyy') + rnum, 'dy','NLS_DATE_LANGUAGE=AMERICAN')
not in ('sat','sun')) as delivery_date from t_av_change_requests where id=104

Still it generates random dates. May i know where I'm going wrong?