Specify the number of days to keep the Oracle log files on the primary server. If this value (in days) is reached, the log files will be removed from the server.
To disable this setting, set to 0, if you are not sure, set to 7.
Make sure you have sufficient disk space to store the archive logs.
See also the ARCHDEST parameter under the standby server variable section for more detail
DAYS_TO_KEEP_ARCHSOURCE = 7