I was able to get a room once at 8:30am, but I had to take a smoking optional room, I would've had to wait for a non-smoking room. Of course, that was years ago, prior to the current all non-smoking policy.
There have been times where we've waited until 5:30pm for a room, though. You just never know what may or may not be available when you arrive. Plan for late check-in and hope for an earlier opening.