It really depends. I do know that if it is ready they will let you check in. I have a friend who went around the same time (a couple of weeks ago) that we did. I believe they were able to check in Really early, around 7:00 AM or something like that. We checked in as soon as we got there and our room was also available. As soon as we checked in we went to our room, looked at our view, then took off to explore the hotel. The Hotel is AMAZING. I really hope on our next vacation we can plan one day at the hotel, one day at the parks, and so on.
Will you be stopping any along the way to sleep, or driving straight in 18 hours? We drove around 20 hours ourself, but I was a wimp and we stopped at two different hotels along the way so we were well rested when we got there.