You are correct; retention times need to support your product registration/marketing activities. So if you plan to secure marketing approval in the EU, your retention times need to meet EU requirements, currently 25 years after closure of the trial. It is common practice for the expected retention time to be clearly shown in the clinical trial agreement with the site so that they understand their obligations right from the outset. The sponsor can provide financial support to ensure document retention but must never have direct access to the ISF or store the records on behalf of the site. Many sponsors use independent third parties to arrange storage on their behalf.
Hope this helps.