Generally schedule changes occur quarterly, although there isn't any set schedule. For a couple of years during the merger, they were happening monthly. The best advice is just to monitor your reservations on aa.com for schedule changes, equipment changes, seating changes, etc. I generally do it about once a week.

Also, note that while Southwest rarely made changes after it opened it's schedule, there have been reports in the last several months of schedule changes on them as well. It's best just to monitor all your reservations for all airlines.