How long are log files needed?

I have been told to use ddboost on our SAP/oracle servers.  The issue is the logs are growing.How long do they need to be kept?  What is the proper way to trim them?

-rw-rw-rw- 1 oraprd sapsys 48452673 Jun 14 22:35 backintPRD.log
-rw-rw-rw- 1 root sys 155804694 Jun 15 10:45 ddbmadmin.messages.log
-rw-r--r-- 1 oracle oinstall 427947587 Jun 15 10:48 arch_backintPRD.log

I do trim the backup sets 

/opt/ddbda/bin/ddbmadmin -d -t -e '-1 month' -n saporacle -z /oracle/PRD/121/dbs/sap_oracle_ddbda.utl -c