For the data retention functionality a manual tester needs to remember the search data so that the tester can perform the test cases for the search retention feature.
By Taking an example of a social networking app , being a manual tester you need to remember all the users that you searched for recently.
To check the time period of retention you can take the help from the backend developer so that they can change the time period (from like one year to 10 min) for testing purpose.
Even if you delete the search history and then you start typing the already entered search result the related result should pop on the first location of the search result. Data retention policies concern what data should be stored or archived, where that should happen, and for exactly how long. Once the retention time period for a particular data set expires, it can be deleted or moved as historical data to secondary or tertiary storage, depending on the requirement