ICO guidance is that organisations should only keep the images for as long as necessary to meet the purpose of recording them. The DPA does not prescribe any specific minimum or maximum retention periods which apply to all systems or footage.A retention time of 31 days has commonly been used for most CCTV applications and is still recommended by police.
According to the National CCTV Strategy report from the Home Office in the U.K., it has long been accepted that CCTV recordings should routinely be kept between 28 and 31 days before being recorded over. This time period allows the police the opportunity to recover CCTV evidence and respond to lines of enquiry that may not have been known at the time an incident was reported.
From an insurance perspective, CCTV can certainly help in defending claims. Therefore you could copy footage of such incidents and retain for a period for 3 years after the alleged incident.
Citations and References
ICO CCTV code of practice Revised edition 2008 http://ico.org.uk/for_the_public/topic_specific_guides/~/media/documents/library/Data_Protection/Detailed_specialist_guides/ICO_CCTVFINAL_2301.ashx
National CCTV Strategy report from the Home Office https://www.cctvusergroup.com/downloads/file/Home%20office%20strategy.pdf