The Hold for the reservation is usually until 11:59PM (not sure about time zones)of the next day (not always - could be current day or even longer if it is a fully refundable fare, But technically the fare is only guaranteed for 24 hours.
But most cases AA honors the fare up to the 11:59PM the next day.
Your AA email will tell u when the reservation hold expires.
The exception on a fare (not the reservation hold) is if the fare expiries today due to a sale or an Advance Purchase requirement (ie 7 days)than the fare is not guaranteed for 24 hours.