You can use "setTimeout()" to trigger an arbitrary function after x
amount of seconds.
See:
http://www.w3schools.com/js/js_timing.asp
So something like ...
setTimeout('$.facebox.close()', 2000);
... would execute $.facebox.close() after 2 seconds.
The close trigger you mentioned is just a way to bind something else
to "Trigger" the close facebox event , which is : $.facebox.close().
So depending on your code, you can do it any number of ways, but I
think the setTimeout function is what you are after.
Hope this helps.