Hi Nick,
It isn't a requirement in the standard but noted as a good practice in BRCGS Issue 9 Guidance:
Good practice is to ensure that a programme of ongoing verification and monitoring is in place to demonstrate that claims are consistently met.
So, I agree with Evans X and would be sampling annually plus when there are any changes (ingredients, suppliers, recipe, process). I would hope that you have covered off Charles' point in your validation.
Your BRCGS auditor may think you should be sampling more frequently but I would be confident in defending that position, you are covering off a guide not a requirement and have probably got bigger fish to fry!
Kind regards,
Tony