WPF: Mouse left button click and hold down handling

I'd look at a slightly different approach that doesn't require the overhead of setting up a timer. If you handle both the mouse down and up events, and store the time when the button was pressed, you can compare it to the time when the mouse is released and decide what to do.

private DateTime mousePressed;

private void txtBlockOnOff_MouseLeftButtonDown(object sender, MouseButtonEventArgs e)
{
    mousePressed = DateTime.Now;
}

private void txtBlockOnOff_MouseLeftButtonUp(object sender, MouseButtonEventArgs e)
{
    // button released

    TimeSpan difference = DateTime.Now - mousePressed;

    if (difference.TotalSeconds >= 3)
    {
        // long press
    }
    else
    {
        // short press
    }
}

There is an obvious reason why this event is not available in MouseListener: it's going to spam you with events such that everything is slowed down to a halt. Do you want to receive this event every second, every ms, or even more often? If you need that you'll have to do it yourself.

For stearing this process you of course need mousePressed and mouseReleased to determine whether a button is currently held down. Then you need to run some kind of loop that generates the corresponding events you'd like to have.

You might also want to work via polling, i.e. extend your MouseListener class such that it can tell you whether a button is still held down, and whereever you need those events you can actively poll for the button. It depends on how you want to use these events, which approach is more suitable.