Anyone have any experience adding Point Shield to reservations after 24 hour reservation grace period?

Anyone have any experience adding Point Shield to reservations after 24 hour reservation grace period? My dad called to cancel a reservation the other day a couple of days after making it and the representative offered to check with her supervisor to see if they could add Point Shield. It wasn't a lot of points, so he declined, not knowing if they would do this more than once and wanting to save the option for the future. What's the longest after a reservation anyone has been able to add Point Shield? Is this something that can be done more than once?