All but 1 time in the past 20 trips has our room not been ready and that was in April with connecting rooms. The way we used to go was this-
I work til 4 am on Saturday night, come home wake kids, (car was loaded up before I went to work the night before) kids get dressed as I shower, grab pillows and last minute items. We are on the road by 5-5:30. After getting off FL turnpike we fill up with gas. Arrive at our hotel about 9 and check-in. As I said we have always been lucky about our room being ready. So we unpack and sometimes either I take a nap then (best thing is when I wake I am in Disney World) or we head to a park for a few hours.
Depending where you are staying and when has alot to do if the room will be ready or not. Plus how many requests you have also is a deciding factor.