private bool _upDownKeyIsPressed = false;

public MainWindow()
{
    RefreshTimer();
}

public void RefreshTimer()
{
    var refreshTimer = new Timer();
    refreshTimer.Elapsed += (sender, e) => RefreshPlotList(sender, e);
    refreshTimer.Elapsed += (sender, e) => RefreshWatchList(sender, e);
    refreshTimer.Interval = 2000;
    if (_upDownKeyIsPressed)
    {
        refreshTimer.Enabled = true;
    } else
    {
        refreshTimer.Enabled = false;
    }
}

private void KeyUpPress(KeyEventArgs e)
{
    if (e.Key == Key.Up || e.Key == Key.Down)
    {
        _upDownKeyIsPressed = true;
        Console.WriteLine(_upDownKeyIsPressed.ToString());
    }
}

private void KeyDownPress(KeyEventArgs e)
{
    if (e.Key == Key.Up || e.Key == Key.Down)
    {
        _upDownKeyIsPressed = false;
        Console.WriteLine(_upDownKeyIsPressed.ToString());
    }
}