72 hours, because you're now working under package policies. Once you add a dining plan, your reservation is not a room-only reservation even if you have an annual pass. It becomes a ticketless package and is subject to all package payment and cancellation policies. So if you add the dining plan 3 days before you arrive, you must pay the balance on the reservation at the same time you add the dining plan.
You should also be on notice that this will effectively result in the cancellation of your room only reservation and the rebooking of your reservation as a package. So if you do it within 5 days of arrival you may be subject to package cancellation restrictions. This has happened to other guests. Clear it with the CM before you do anything, if you are making this change within about a week of arrival.