select DATEDIFF(mi, '19000101 09:00:00', '19000101 18:00:00')

declare @dt_start  datetime = '20120101'
      , @dt_finish datetime = '20121101';

with cte
as
(
select cast( @dt_start as date ) dt
union all
select dateadd ( d, 1, dt ) dt from cte
where dt < cast(  @dt_finish + 1 as date )
)
select sum( mi ) sum_mi
from
(
select dt
, case when @dt_start > dt 
       then case when datediff( mi, dt, '20120809 18:00' ) < 0 then 0
            else datediff( mi, dt, '20120809 18:00' ) end
            - case when datediff( mi, dt, '20120809 09:00' ) < 0 then 0 
            else datediff( mi, dt, '20120809 09:00' )
            end
       when @dt_finish < dt 
       then case when datediff( mi, @dt_finish, '20120809 18:00' ) < 0 then 0
            else datediff( mi, @dt_finish, '20120809 18:00' ) end
            - case when datediff( mi, @dt_finish, '20120809 09:00' ) < 0 then 0 
            else datediff( mi, @dt_finish, '20120809 09:00' )
            end
  else 540           
  end as [mi]
from cte
where datediff( d, 0, dt )%7 < 5
) t
option ( maxrecursion 0 )

declare @dt_start  datetime = '20120808 15:00'
      , @dt_finish datetime = '20120810 17:00';

with cte
as
(
select cast( @dt_start as date ) dt
union all
select dateadd ( d, 1, dt ) dt from cte
where dt <= cast(  @dt_finish as date )
)
select dt
, case when @dt_start > dt 
       then case when datediff( mi, @dt_start, dateadd( hh, 18, cast( dt as datetime ) ) ) < 0 then 0
            else datediff( mi, @dt_start, dateadd( hh, 18, cast( dt as datetime ) ) ) end
            - case when datediff( mi, @dt_start, dateadd( hh, 9, cast( dt as datetime ) ) ) < 0 then 0 
            else datediff( mi, @dt_start, dateadd( hh, 9, cast( dt as datetime ) ) )
            end
       when cast( @dt_finish as date ) = dt 
       then case when datediff( mi, dateadd( hh, 09, cast( dt as datetime ) ), @dt_finish ) < 0 then 0
            else datediff( mi, dateadd( hh, 09, cast( dt as datetime ) ), @dt_finish ) end
            - case when datediff( mi, dateadd( hh, 19, cast( dt as datetime ) ), @dt_finish ) < 0 then 0 
            else datediff( mi, dateadd( hh, 18, cast( dt as datetime ) ), @dt_finish )
            end
  else 540           
  end as [mi]
from cte
where datediff( d, 0, dt )%7 < 5
option ( maxrecursion 0 )

declare @dt_start  datetime
      , @dt_finish datetime

select @dt_start = '20120808 15:00', @dt_finish = '20120810 17:00'

select sum( mi ) sum_mi
from
(
select * 
, case when @dt_start > dt 
       then case when datediff( mi, @dt_start, dateadd( hh, 18, dt ) ) < 0 then 0
            else datediff( mi, @dt_start, dateadd( hh, 18, dt ) ) end
            - case when datediff( mi, @dt_start, dateadd( hh, 9, dt ) ) < 0 then 0 
            else datediff( mi, @dt_start, dateadd( hh, 9, dt ) )
            end
       when dateadd( d, datediff( d, 0, @dt_finish ), 0 ) = dt 
       then case when datediff( mi, dateadd( hh, 09, dt ), @dt_finish ) < 0 then 0
            else datediff( mi, dateadd( hh, 09, dt ), @dt_finish ) end
            - case when datediff( mi, dateadd( hh, 19, dt ), @dt_finish ) < 0 then 0 
            else datediff( mi, dateadd( hh, 18, dt ), @dt_finish )
            end
  else 540           
  end as [mi]
from
(
select dateadd( d, number, dateadd( d, datediff( d, 0, @dt_start ), 0 ) ) dt
from master..spt_values
where type = 'P'
) t
where dt <= dateadd( d, datediff( d, 0, @dt_finish ), 0 ) 
and datediff( d, 0, dt )%7 < 5
) t