I currently work in a project where we add and delete thousands of list items. My recycle bin had over 30.000 items and I had to clean it. Cleaning out that amount of items the standard way would take hours. I googled a bit and found out thet there is a hidden feature in the recycle bin. Go to the recycle bin page, type javascript:emptyItems(); in the address bar and hit enter. You will get a confirmation dialog and if you click yes the entire recycle bin will get purged! This works as well for recycle bins on individual sites and for the site collection recycle bin, on both SharePoint 2010 and 2013. Why Microsoft have decided to hide this gem is beyond me.
Yet another day in SharePoint paradise...
