I believe it's been 4 hours, however I did experience a 3 hour check-in time once, however I was later informed that it was 4 hours, and the agent wasn't correct (well after I had waited an additional hour pre-security).
aa.com states:
Airports with checked bag limits
You can't check bags more than 4 hours before departure when flying from:
- Denver (DEN)
- Fort Lauderdale (FLL)
- Honolulu (HNL)
- Kahului (OGG)
- Las Vegas (LAS)
- Orlando (MCO)
- Portland, Oregon (PDX)
- Salt Lake City (SLC)
- Seattle (SEA)