The text on AA.com says "For flights departing Hawaii and the U.S. Virgin Islands, recommended check-in time is at least two hours prior to scheduled departure". I'd venture to guess that this is just related to the fact that most people at those airports are infrequent travelers and take a long time to process. Note that this is a guees.

Personally, given that there are 10 flights STT-SJU and you're not connecting anywhere, I'd show up at the time I'd show up at any other airport.