Seven years.

See the paragraph 13 of the [Crown Court Records Retention and Disposition Schedule](https://assets.publishing.service.gov.uk/government/uploads/system/uploads/attachment_data/file/911194/crown-court-rrds.docx):

> 13 Recordings of trials -- Keep for **7 years** and then destroy

For awareness, and completeness, the list of retention schedules for other courts and tribunals may be found [here](https://www.gov.uk/government/publications/record-retention-and-disposition-schedules).