How long should we allow for processing through FCO on our return flight (FCO to ATL via Amsterdam on KLM)? Thanks.
Processing what? It's a EU Schengen Treaty area flight. They don't even check your passport. Just arrive to check-in about 1 hr prior to departure.
Before you decide to give yourself only an hour, you might want to read Ron In Rome's account of one experience at FCO for a flight going to Paris (a Schengen country). Another chaotic morning at FCO The short version is that they arrived at the Airport at 04:50 for a 07:10 flight to Paris, and yet they barely made it. But then again, they had luggage that they had to check, and that was the main hold up.
My memory from 2008 was that we didn't have a moment to spare-with only carryon luggage- and we allowed 2 1/2 hours. Of course I believe they were finishing a major overhaul of the "to US" terminal. And my memory ain't what it used to be.
Since in another post you mentioned that you are going to Tuscany only, then change your ticket and depart from Florence instead to go to Amsterdam. Never had a problem in Florence arriving one hour prior to departure. I haven't used FCO on an intra EU flight for a few years now.