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That’s the right to full erasure. Revoking consent for processing your data for marketing purposes should be possible at any time and as easy as giving consent:

“The data subject shall have the right to withdraw his or her consent at any time. The withdrawal of consent shall not affect the lawfulness of processing based on consent before its withdrawal. Prior to giving consent, the data subject shall be informed thereof. It shall be as easy to withdraw as to give consent.” (Article 7(3) GDPR)

That does not mean that you cannot take a few days to process the revocation, but it does mean that you should handle it as soon as possible and the burden is on you as data processor to ensure you still have valid (non-revoked) consent before sending another mail.