...
letzte:longint;
  begin
   Timer1.Interval := 300000;  //=300000 ms = 300 s = 5 m, you can change timer interval
    i:=1; 
   Timer1.Enabled := True;  //=call TForm1.Timer1Timer
  //get actual data and store it in "str"
...
 
 
//This procedure will be execute every 5 minutes, so popup appears after 10X5 minutes
procedure TForm1.Timer1Timer(Sender: TObject);
begin
  if (i=10) then
  begin
    Timer1.Enabled:=false;
    i:=0;
    ShowMessage('Please get up and relax your eyes');
  end;
  Inc(i);
end;