Up until recently I was under the impression (like may others with me) that the timeframe you can configure for the recycle bin in the Central Administration page was for the first stage only. When that period was expired, SharePoint would move the content to the second stage recycle bin.
Last week I have been talking with a Microsoft employee who told me something different. According to him, SharePoint deletes the items from the recycle bin after this period, both from the first AND second stage recycle bin. The only way documents get specifically moved to the second stage is when a users cleans his recycle bin.