We had free dining going on and only 90 day out reservations for our early December trip, but I was able to book a later time without a problem. It was much harder to get anything before 9am. About a week or two before our trip I was able to snag an 8:50am ressie. We got to the park early and were seated by 8:30am! I would have preferred the 8:05 time, but what we got was better than where we started (9:50am).
My advice regardless of the time of year you are going: the earlier ADR times will be hardest to get. If you want a early ADR make it a higher priority on your list of ADR's. Tusker House was in my top 4 of 18 and that 9:50am time was the best available and I was on the internet logged in and ready to go at 6am EST.
Good luck with getting the ressies you want! Have a great trip!