6 hours is the max allowed IF there is a flight within that time. Up to 24 hours is allowed and won't count as a stopover if that's the only possible connection.

That said I and some other AA FFers booking a OneWorld award have been succesful in getting a longer layover when the earlier flight had no award iventory. It's not suppose to work that way but most times than not it does. If there is award inventory on the earier flight then you have to take that flight.