How long on average does it take to get through customs at FCO? Plane lands at @9:30, no luggage, just a carry on
Customs is a breeze. There is no paperwork. Unless you have something to declare you just walk right through. The delay could come at Passport Control.
The non EU line for passport control (immigration) is sometimes long (when I don't travel with my wife I opt to use my Italian passport for that reason, since the EU line has no line). However it's not going to take you more than 10-15 min to go through. When it gets really bad because of simultaneous arrivals I've experienced that the Polizia di Frontiera opens the EU line to US citizens as well (I guess citizens of the US are not considered a "risk"). There is no real customs line, you just walk through. My longest and most frustrating waits at FCO have been at the carousel waiting for my checked in luggage. Therefore your choice of traveling with carry on only is a wise one. Stick with that choice.