Timer tick when the for loop is completed

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VB

How can I tick the timer when the for loop is completed? I don't want to run the timer every X minute, I want to run the timer when the button is clicked and then just tick the timer again when the for loop is completed..

 For Each row As DataRow In MTProcessTable.Rows
  Try
            If checkKeyHelp(process.datain) Then
                msg = msgTable.Rows(1)(1).ToString()
                MsgBox(msg)
                writeMsg("1 MO help at ")

            ElseIf checkKeyInfo(process.datain) Then
                msg = msgTable.Rows(4)(1).ToString()
                MsgBox(msg)
                writeMsg("1 MO INFO at ")
            Else

                MOTable = selectMO(process.mo, process.mt)
                moRowNb = MOTable.Rows.Count()
                MO = New MO_class
                If moRowNb <> 0 Then
                    MOrow = MOTable.Rows(0)
                    MO.newuser = MOrow("newuser")
                    MO.sim_id = MOrow("sim_id")
                End If
       Catch ex As Exception
            logFile("executeTimer ----" & ex.Message)
            updateProcessed(process.id, ex.Message)
        Finally
            updateProcessed(process.id, msg)
        End Try
 Next row

Private Sub start_btn_Click(sender As System.Object, e As System.EventArgs) Handles start_btn.Click
    Timer1.Enabled = True
    tm.StartTimer()
End Sub

when the timer starts, i'm filling a datatable and i'm executing some action on every row in the for loop and when the for loop is finished the function should start all over again and datatable is refilled with a different number of rows...so i can't know the time needed for the function to be completed because everytime i have different number of rows that's why i just need to tick the timer once the function has executed all the rows in the datatable

any help?