Out of curiosity are you referring to the HTTP Access logs or the system logs like the pvac.log?
The pvac.log and comm_srv.log is stored in /var/log/wa and is zipped on a daily basis and archived for 7 days on the system.
HTTP hit logs or access logs are stored in /var/wa/log/access and these are stored for 30 days on the system.
Looking at these directories you should see daily files as *.gz
It is definitely recommended that these files be transferred to external storage especially if they are going to be needed long term.