From AA.com
If you haven't joined yet there is no retroactivity. If you have joined, the last year's worth can be submitted with the data per flight. Good luck. This is a tough thing to do after the fact.
9. Request Mileage Credit
Q: How do I request mileage credit that hasn't been applied to my account?
A: Select 'AAdvantage' from the side menu and then 'Request Mileage Credit.' For AA flight credit, fill out the form with applicable information and submit. This page also contains information on where to send mileage credit requests for non-airline participants such as hotels, car rentals, and special services.
Q: How long should I wait before I request my mileage credit?
A: You should wait a minimum of 15 days from the completion date of your trip. For mileage credit transactions with other partners, allow a minimum of 30 days from the transaction. <I>Any request for mileage not credited to your account must be requested within 12 months of the activity</I>. <B>Activity prior to your enrollment date is not eligible for mileage credit</B>.
Q: Once I request the mileage credit, how long will it take to be applied to my account?
A: Allow up to 30 days for miles to be posted to your account.