To utilize a Retention Policy for a data extension that stores web behavior data, two methods are possible. One is to delete data from the data extension prior to twelve months ago, then configure a Retention Period via Contact Builder. This will allow the data extension to keep only the data from the previous twelve months and delete any older data automatically. The other method is to replace the current data extension with a new data extension configured with a Retention Period. This will create a new data extension with the same fields and settings as the old one, but with a Retention Period enabled. Reference: https://help.salesforce.com/s/articleView?id=sf.mc_co_data_retention_policies.htm&type=5 https://help.salesforce.com/s/articleView?id=sf.mc_co_create_a_new_data_extension_from_an_existing_data_extension.htm&type=5