My parents must be running around at Atlanta airport. Their flight DL 0125 from Brussels arrived at 2:17 pm and their connecting flight DL 0590 to Pittsburgh is schedule to leave at 5:15 pm.
That should give them ample time to get through immigration, customs, collect their bags, re-check in the bags, and get to the gate where their flight to Pittsburgh departs from.

I haven't had a phone call yet and if all goes well they should be through immigration and customs in half an hour from now.
Rough estimate of course :-)

I'm leaving at 6 pm to pick them up at the airport so 3 hours left.