When doing online check-in, if I want my room to be available before check-in time I usually put in my estimated arrival as an hour before my expected actual arrival. I always receive a text at that estimated arrival time, and often it says my room isn't quite ready yet, but I feel like they really do their best so typically I receive a text within the next hour saying it's ready.

Always keep in mind though if you are visiting at a busier time of year, that you may not get in before check-in time. As an example: I've had our DVC room be ready at 11am in mid-January but not until exactly 4pm during the Christmas season. And in general, I've found my DVC rooms to generally be ready later than my non-DVC rooms. I visit more frequently than the amount of points I have so I'm usually there at least once per year on cash rather than points.