I been on the phone with Disney Dining five different times about your question and gotten different answers from five different reservation agents. Arrrgh!
I now
think the official rule is that 90 days before your check-in at Port Orleans, you will be able to make eight days of advance dining reservations (ADRs). Then, 90 days before your check-in at the WIlderness Lodge, you will be able to make dining reservations for the remaining seven days of your trip.
However, in practice, you are likely to encounter some loose interpretations of this rule depending on who you happen to get on the phone. You may find that your reservationist will let you make 10 days of reservations from the start, or even let you make reservations for your entire 15 day stay. (This last circumstance is less likely, but it is possible.)
My advice to you would that if you don't get the answer you want the first time, try calling back and see if you find an agent that will be more accommodating.
You can find an answer to a related question answered by former Panelist Bret,
HERE (Note that at the time he wrote, dining reservations could be made 180 days in advance. The current window is 90 days, but the substance of the answer is the same.)
Other questions you may be interested are
HERE and
HERE.
Sorry I can't provide a more concrete answer. My best advice is really to call, be nice to the agent on the phone, and keep your fingers crossed for making as many reservations as possible.
Good luck!