Does anyone know if I got the time to start up the 747 before the scenery reverts back to default?
The Trial time doesn't start until you enter in the O'Hare area so, if you don't have a ready made situation with the airplane started, you might want to make the start-up sequence on a different airport, then warp to O'Hare using the World->Airport menu.
This might be useful also if you want to plan a flight into O'Hare. The trial time starts about 8-9 nm outside the airport perimeter so, it should be enough to allow you to land.
Also, don't forget there's no limit on the number of times you can restart FSX, and getting another trial. You simply can't fly for more than 5-6 minutes at time, but there's no limit of how many trial periods you have, and there's no expiration date either.