Copy and paste this into your address bar and it will update itself every 5000 milliseconds (aka 5 seconds, meant to be hint on what that "5000" there means, and you can change it as well).
javascript:setInterval('lx();',5000);alert('Shoutbox auto-update activated!');function lx(){$('chat_message_spinner').show();new Ajax.Request('/chat_rooms/1/chat_messages',{asynchronous:true,evalScripts:true,method:'get',onComplete:function(request){$('chat_message_spinner').hide();},parameters:'last_chat_message_id='+$('last_chat_message_id').value});return false;}
Note that it will only work until you refresh or move away from the page.