pre:

Sub DoTheThing()
    Dim targCol As String
    targCol = "Z"   'Change this to a column of your choice.
    Dim startRow As Integer
    startRow = 2    'Change this to the first row with data.
    
    Dim ts1 As String
    Dim ts2 As String
    Dim lts1 As Integer
    Dim lts2 As Integer
    
    Dim numOnly As String
    Dim check As Boolean
    
    Dim hourCount As Double
    
    Dim ws As Worksheet
    Set ws = ActiveSheet
    
    For i = startRow To 10000000
        If ws.Cells(i, 1) = "" Then Exit For
        
        check = True
        hourCount = 0
        
        If WorksheetFunction.CountA(ws.Range("C" & i & ":P" & i)) mod 2 = 0 Then
            'Matching pairs. Next!
            For j = 3 To 15 Step 3  'Getting them in pairs.
                ts1 = WorksheetFunction.Text(ws.Cells(i, j), "hh:mm")
                ts2 = WorksheetFunction.Text(ws.Cells(i, j + 1), "hh:mm")
                
                'I could handle this so much better, but I *really* need to be somewhere else right now.
                If ts1 = "" Then ts1 = "00:00"
                If ts2 = "" Then ts2 = "00:00"

                'Clean yo data.
                numOnly = ""
                For k = 1 To Len(ts1)
                    If IsNumeric(Mid(ts1, k, 1)) Then
                        numOnly = numOnly & Mid(ts1, k, 1)
                    End If
                Next k
                ts1 = numOnly
                
                numOnly = ""
                For k = 1 To Len(ts2)
                    If IsNumeric(Mid(ts2, k, 1)) Then
                        numOnly = numOnly & Mid(ts2, k, 1)
                    End If
                Next k
                ts2 = numOnly
                
                'Is it a simple one?
                If (Len(ts1) = 4) And (Len(ts2) = 4) Then
                    'Yup
                    hourCount = hourCount + TimeCounter(ts1, ts2)
                Else
                    'Nope
                    If (Len(ts1) <> Len(ts2)) Or (Len(ts1) Mod 4 <> 0) Or (Len(ts2) Mod 4 <> 0) Then
                        check = False
                        ws.Range(targCol & i) = "Data error!"
                        Exit For
                    End If
                    
                    lts1 = (Len(ts1) / 4) - 1
                    lts2 = (Len(ts2) / 4) - 1
                    
                    ReDim tArray1(lts1) As String
                    ReDim tArray2(lts2) As String
                    
                    'Loop through the numbers to get matching pairs.
                    For k = 0 To UBound(tArray1)
                        tArray1(k) = Mid(ts1, (k * 4) + 1, 4)
                        tArray2(k) = Mid(ts2, (k * 4) + 1, 4)
                    Next k
                    
                    For k = 0 To UBound(tArray1)
                        hourCount = hourCount + TimeCounter(tArray1(k), tArray2(k))
                    Next k
                End If
            Next j
            
            If check Then ws.Range(targCol & i) = hourCount
        Else
            'Not enough data.
            ws.Range(targCol & i) = "Not enough data!"
        End If
    Next i
    
End Sub

Function TimeCounter(time1 As String, time2 As String) As Double
    'Returns the number of hours.
    Dim startHour As Integer
    Dim endHour As Integer
    Dim minutes As Integer
    
    minutes = 0
    
    startHour = CInt(Left(time1, 2))
    endHour = CInt(Left(time2, 2))
    If Right(time1, 2) <> "00" Then
        minutes = minutes + 60 - CInt(Right(time1, 2))
        startHour = startHour + 1
    End If
    
    minutes = minutes + CInt(Right(time2, 2))
    
    TimeCounter = endHour - startHour + (minutes / 60)
End Function