Our earliest was 6:30a during a "continuing stay." We had 10 nights booked in the same resort and unit type through two different reservations: 7 night inbound exchange (II) and 3 nights on points. The first check-in was perfectly normal (sometime after 4PM, standard for us given flight schedules) -- and the 2nd check-in was performed
automagically by the resort staff. Our new room keys and check-in paperwork was slipped under our door at 6:30a on our final morning of the first reservation. The resort allowed us to stay in the same room ... easy, breezy!