It varies. I believe there is a thread somewhere here on the Transportation Board that lists out when JB has extended in the past if you want to search for it. Someone might also have the link handy as well.
If you are not already on JB's mailing list go to the JB website and scroll all the way down and on the right is a link to sign up. Two days before they usually send out an email about the extended schedule. At that time they allow those booking a JB vacation get first crack at booking airfare by being able to book a day early. By booking a mock vacation you can find out what the flight times will be. You also maybe get a good idea at how much the flights are IIRC.