Delete Entries in Application Log (R/3) 
Use
You can use the report RDELALOG to delete records from the application log in R/3. The records are not automatically deleted from the system. To avoid the database from overflowing, we recommend that you delete the records at regular intervals.
If required, you can also schedule an appropriate background processing job.

This report is valid for all releases that have an
Choose:
In APO, you can access a similar function using transaction SLG2.
See
Deleting Records 
If you do not make any entries in the date and time fields, all records that are older than two weeks are automatically deleted.