The rules are 4 p.m. We went on a Sunday in Oct and got there at noon and we were told we could not check in until 4 p.m. They gave us a phone number to see what room we were assigned.
Actually DVC changed the wording a few years ago from "check in is at 4 pm" to "check in is after 4 pm". If the room is ready and they know it, they will give you the room as early at about 4:30 to 5 am when the computers change over.
I totally got hooked up last week, we were changing from SSR to BCV and we got a room at 10:30 AM! I was so excited to be settled before our 10:55 breakfast ressie! So hey you never know.
We have been able to get our villa before 4PM. This all depneds on the season & occupancy. If times are slow there is a much better chance of early room availability.
We checked in last Monday around 11am and we told they would call our cell phone when room was ready. We got a call around 3:30pm that it was ready. The next day we moved to a 1br and checked-in at 8am before we headed to the parks. We got a call around 11am that our room was ready. I think it is just hit or miss like other posters have stated.