180 days in advance. However, I am not a DVC person so I don't know if these people have something special for them as far as ADR timing. Assuming you are a "regular" person like me than it is 180 days prior to arrival. Good luck!
According to the dining calculator on this site, your 180 days would be March 22. On-line opens at 6 a.m. eastern time and phones open at 7 a.m. eastern time.
We are checking in at Port Orleans French Quarter the same day you are. When I made our resort reservations with free dining (bounce back offer last September) I was told to make dining reservations on March 22nd! Good luck getting what you want!