The default retention period in EM Cloud Control for closed alerts is 180 days, You can modify this from inside EM Cloud Control as follows:

1) Navigate: Setup > Manage Cloud Control > Repository

This brings up the Management Services and Repository page)

2) Click on the Schema tab

• In the ‘Purge Policies’ section, select ALERTS and click on Modify.
• Also select EVENTS and click on Modify.

The purging is done via a DBMS Scheduler job in the repository database.

Be aware that significant changes from the default values for retention in EM data may require a change in the resource allocation to the repository database.
The standard operating resource requirements and tuning examples can be found here (for EM 13.2 Cloud Control):