DECLARE	@timeend TIME = '17:00'
DECLARE	@timestart TIME = '08:30'
DECLARE @daylength INT = DATEDIFF(mi, @timestart, @timeend)
DECLARE	@StartDate DATETIME= '2012-05-10 8:30:00.000';
DECLARE	@EndDate DATETIME= '2012-05-14 15:00:00.000';

SELECT
	(DATEDIFF(d, @StartDate, @enddate) * @daylength)  --TotalMinutes,
	- (DATEDIFF(week, @StartDate, @enddate) * 2 * @daylength)  --Weekend
    - (@daylength - DATEDIFF(mi, @timestart, CAST(@EndDate AS TIME))) --LastDay
    - DATEDIFF(mi,@timeend, CAST(@startdate AS TIME) ) -- First Day