Even 24 hours out, you can get a lot of things. If I wanted a table for 4, dinner, tomorrow between 5-7PM, I can get Chef Mickey, Cali Grill, Artist Point, Boma, Biergarten, Garden Grill, even Jungle Book dining packages. I could get an early or late BOG (before 5), but not a 5-7PM.
The majors I can't get 24 hours out are what you'd expect: CRT, Crystal Palace, Ohana, V&A. Couple of Disney Springs restaurants that I bet I could get on Open Table if I wanted to try.
Now, sure, this is one Wednesday in July leading to one Thursday in July, but it gives you an idea.