Mit szólnál ehhez?

Function BackslashHelye(s As String) As Byte
Dim i As Integer

For i = Len(s) To 1 Step -1
If Mid(s, i, 1) = "" Then
BackslashHelye = i
Exit For
End If
Next i

End Function

Sub tesz()

MsgBox BackslashHelye("C:\elso_konyvtar\masodik_konyvtar\.......x-ik_konyvtar\file_name.xls")

End Sub