How long, on average, does it take to walk from ITA international arrivals to ITA domestic departure? I will be traveling on ITA from JFK to Florence and airline only plans on 1 hour, 20 min leeway between arrival and departure. I know that I'll have to go through customs too. Airline says "plenty" of time...truly?
You will make as long as JFK to Rome departs and arrives on time. The airline is obligated to put you on the next available flight with space available if the first flight is late causing you to miss the connection. You are not going to be the only one on that flight with a connection to make. You go through immigration passport control in Rome and pass customs after baggage claim in Florence.
Why did you pay for the ticket after seeing that 1 hour and 20 minutes is a legal connection time? When you buy a ticket on an airline you have 24 hours to cancel and get your money back without penalty if you have 2nd thoughts.
If your flights are all on the same ticket and the one from JFK is on time, you will be fine. You’ll go thru passport control not customs on your way to your connecting gate.