There is no standard answer that would mean much for your specific case, although there are probably going to be people with experience on those flights who can provide a little insight. In the meantime, you might want to run your itinerary through a tool to see how many seats are currently open that you could potentially find yourself sitting in:
http://www.flyaow.com/classamex.htm
It's possible that you can at least get a little bit of a feel for your chances...