Private fUnsavedChanges As Boolean
Private strPath As String

Private Sub cmdOpen_Click()
    CommonDialog1.Flags = cdlOFNFileMustExist
    CommonDialog1.Filter = "Text Files|*.txt"
    CommonDialog1.ShowOpen
    
    strPath = CommonDialog1.FileName

    RichTextBox1.LoadFile strPath, rtfText
End Sub

Private Sub RichTextBox1_Change()
    fUnsavedChanges = True
End Sub

Private Sub Timer1_Timer()
    If fUnsavedChanges Then
        RichTextBox1.SaveFile strPath, rtfText
    End If
    fUnsavedChanges = False
End Sub