As part of your Research Data Management Plan, you would have determined the minimum retention period for your data. Once that period has expired, it doesn’t mean that you automatically have to destroy your data. You may wish to retain all your research data for as long as possible without breaching the legal or ethical requirements of your research project. Please note that you may need to explain in your Human Ethics application why you wish to retain data indefinitely.
There are circumstances where you may need to destroy your data. For example, if the Participant Information Statement for your project specifically stated that all research data generated would be destroyed after a certain period of time, you will need to destroy your data accordingly. In this case, the destruction process must be secure and irreversible, and include all your data - both electronic and physical - and include all master copies and any data backups. Please be aware that best practice for electronic data destruction changes, so it's probably best not to put anything specific, just that it will be done in accordance with best ICT practice.
The documents linked below provide further information: