Easiest thing to do is to back out the number of hours between business days for the number of business days and then 24 hours for each non-business day. This is no simple matter but below is a function I wrote some time ago that calculates the number of weekdays between two dates. Note the third parameter countfirstday (True: monday to monday is 8 days or false: 7). I hope this gets you moving in the right direction.
Function weekDayDiff(StartDate As Date, EndDate As Date, CountFirstDay As Boolean) As Long
Dim dtStartAdj
Dim dtEndAdj
Dim lngWeeks As Long
Dim lngStartDay As Long
Dim lngEndDay As Long
lngStartDay = WeekDay(StartDate, vbSunday)
lngEndDay = WeekDay(EndDate, vbSunday)
dtStartAdj = StartDate - (lngStartDay - vbSunday)
dtEndAdj = EndDate - (lngEndDay - vbSunday)
lngWeeks = DateDiff("w", dtStartAdj, dtEndAdj, vbSunday, vbFirstJan1)
'comment Block1: (Friday to Monday is 1 not a weekend day to Monday) Function acts weird without below
If lngStartDay = vbSunday Then
lngStartDay = lngStartDay + 1
End If
If lngEndDay = vbSaturday Then
lngEndDay = lngEndDay - 1 'Saturdays don't count
End If
'End Comment Block1
weekDayDiff = lngWeeks * 5 - lngStartDay + lngEndDay + Abs(CountFirstDay) '+ vbSunday -vbSunday
End Function