Per AA's international tarriff, which governs interline travel such as your example (I've flown this fare a couple of times, it's a great deal!):
The period of validity for transportation will be one year from the date on which transportation commences at the point of origin designated on the original ticket, or if no portion of the ticket is used, from the date of issuance of the original ticket.
And AA's conditions of carriage (governing AA marketed flights, if you elected to fly BA on the TATL portion):
Unless your ticket indicates otherwise, tickets are valid for transportation for one year from date of initial use, or if unused, for one year from date of purchase.