Retention time will depend on several issues once there is no prescribed time in ISO standards:
*Are there legal or regulated issues? For example, do you need to keep evidence that you tested your product before delivery to the customer for as long as a warranty is valid?
* Are there after sale issues? For example, do you need to keep evidence to help you service customers a long time after delivery? Customers can order you parts or inquire you about parts or specifications long after the warranty period. Do your organization intend to serve them?
* Are there contract issues? For example, does a contract with a customer requires keeping certain records/evidence for a certain number of years?
* Are there internal issues? For example, does your organization has learning or improvement challenges that require a certain organizational memory? That may imply keeping certain records for a certain time.