Most poeple say it only takes like 30 minutes to get from the first terminal to the last terminal (
ATL's international terminal).
HOWEVER- I partially disagree with this.
One time when I was in
ATL the tram thing that transferred you between terminals what shut/broke down.
It look about 35-40 minutes to walk from the first terminal, for me to walk to see the awsome Korean Air 744.
I suggest you leave some leanway for accidents and cases like this- but normally 40 minutes is fine for domestic travel.