Log in to the Symantec Endpoint Protection Manager
Click on the Policy tab
Click on Antivirus and Antispyware
In Antivirus and Antispyware Click on Miscellaneous Tab then Go to Log Handling
There is one Tab "Delete logs older than" Set the the number of days you want to keep antivirus-related events in the logs.
Then Ok.
Apply the same in to all group.
This will help to change the setting on all Clients.
Hope this information will help you.
Keep in touch if any help require.