Reservations at Cinderella's Table are not the high-pressure, call at 7:59:59 exercise they used to be!
I got one (for four of us, for b'fast, for the THursday before Labor Day, I called in June (about 3 months before our trip), and had choices of times and days (within our five day "window")...
I vaguely recall reading somewhere that now that Cinderella's Table does character meals at lunch as well as at b'fast, it's a much easier reservation to get...
Anyway, calling at 180 days I think it's now likely you'll have some choices as to date and time ...
But note: you probably do need to call far in advance...when I tried to change the size of our reservation (from 4 to 5) at about 2 months out, I couldn't--nothing was available...so it's not like you can walk in or even call just a little ahead and get a reservation!
GOod luck...