Seven years.

See the schedule at paragraph 13 of the Word document "Crown Court RRDS" within the government' guidance Record retention and disposition schedules:

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

(I can't link directly to the Word doc from my phone, sorry)

Seven years.

See the paragraph 13 of the Crown Court Records Retention and Disposition Schedule:

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.