[ADMIN] Frequency of archive_cleanup_command

[A J]

Couple of questions:
1. At what frequency is the archive_cleanup_command executed from the 
recovery.conf file on the standby ?
2. If for some reason lets say a WAL file is missing from the archive directory 
but the files after that are present. When the standby tries to restore the 
missing WAL file, does it fail or it simply skips the missing WAL file and goes 
to the next one.

Thanks.