I think it's at least 12 hours. Not sure if there's a distinction between domestic and international flights, though. This past Friday morning, I checked in for both my MEX-MIA (11:50am CST) and MIA-GIG (11:30pm EST) flights at about 10:30am CST. I received boarding passes for both, and my luggage was tagged/checked through to Rio de Janeiro, so I didn't have to claim it in Miami and take it through customs...