Hi Patrice,
If you allow the customer to cancel the booking via the link, they can do it at any time before the check-in date. You can check this article to learn how it works.
If you have a strict cancellation policy, you can disable the native cancellation via the link. Then describe the rules of the pages of each accommodation type, terms, and conditions page, etc. Additionally, you can add a custom page with a custom contact form to let the guests contact you and request to cancel a booking. Thus you will be able to review each cancellation request manually.