Print jobs in /usr/spool/lp not being deleted

Hi All!

Anyone know why files in the /usr/spool/lp directory do not get deleted?
The user guide and web documentation state that the files get
deleted when they are no longer required. But some how my /usr/spool/lp directory does not get deleted.

What tells the lpsrvr that the files are no longer required? where is the setting ?