You need to keep them as per PPAP book
as per standard 7.5.3.2.1 Record retention
Production part approvals, tooling records (including maintenance and ownership), product and process design records, purchase orders (if applicable), or contracts and amendments shall be retained for the length of time that the product is active for production and service
requirements, plus one calendar year, unless otherwise specified by the customer or regulatory agency.
I would say the samples should be for the same period
You want them so if there is a dispute you have evidence