The hours for Resort Airline Check-in (RAC) are 5:00 a.m. to 1:00 p.m. You can check in any time during those hours for a flight on the same day, as long as you complete your RAC check-in at least 3 hours before your scheduled flight departure.
You cannot use RAC the night before your flight.
They used to say the flight had to be between 3 and 12 hours after your RAC check-in, but now the published rules no longer prevent guests from checking in more than 12 hours ahead of time, as long as the flight is that day.
For more details, see
http://disneyworld.disney.go.com/faq/airport-service/.