Guests staying at Disney-owned resorts can call exactly 90 days from the first day of their resort stay and book reservations for that date plus the next 9 days.
This may not work with some of the restaurants that are not owned by Disney, like the ones at the Swan/Dolphin or Downtown Disney. For some of those, you might need to call exactly 90 days from the day you want to eat there. (These restaurants tend not to accept the dining plan, so it's likely you are not planning to book any)