I was doing some site cleanup today and ended up with around 15,000 items in a site collection recycle bin. I started cleaning them out using the GUI. It was SLOW going. There is no way to "Delete All" for the second stage bin. Rather, you have to delete items from the second stage bin one page of items at a time. Here's a quick tip on how to delete everything with PowerShell.
The following code will delete EVERYTHING out of both stages of the site collection recycle bin:
$site = Get-SPSite http://your.portal.url/site_collection Write-Host "Items in recycle bin:" $site.RecycleBin.Count $site.RecycleBin.DeleteAll()