Originally Posted by
MSPeconomist
OP says six hours an ATL, but I wonder whether the "layover" is a connection or a stopover. PreCOVID-19, the line was four hours for domestic and there were also time limits for bags being checked through, even on long connections. IIRC over six hours on a domestic itinerary is considered a stopover now, but OP's six hours could be under or over. It might be useful to look at the fare construction on the ticket to see whether there's a broken fare.
Except for a very few airport you can check bags anytime same day before flight from it. Domestically your bag may even fly before you do. Regardless 6hrs is when you can short check, 12hrs is when you must short check domestically.