Rome airport time needed to check in to local airline to Sicily

I did this in 2024 but can't predict your transit time in Rome. I just planned to eat lunch in the airport and cool my heels for a bit since the international flight to Rome could have been late. Transit turned out to be quick and easy so lunch was leisurely. There are a lot of reasonably priced flights on ITA and other carriers from Rome to Palermo, and Catania to Rome. If you book a fare that allows changes, you could hop onto a different flight if you wanted to.

That is unknowable as nobody can predict whether the flight arrival is timely, how many airliners are arriving at that time potentially clogging up the passport control, the baggage claim time, the check in time at the Ryanair desk the security line to the Ryanair terminal 1. Maybe 3 hours can be enough, maybe not. Many people who choose to do what you do, often go to Sicily the next day(s), after spending at least one night in Rome, then they do the same on the return leg and spend one night at FCO airport before the transatlantic flight.

But I would take the ITA all the way and forget about Ryanair. I understand the brutal early wake up, but in CTA all you need is to be at the check in by 4:30. It's a domestic flight to Rome, so you don't need to be there at 3am. Or maybe ITA allows you to postpone the second leg to SFO to the next day, and you just spend the night in a hotel. In that case you would take the CTA-FCO leg the day before at the time of your choosing, then after a night at the airport hotel, take the long leg FCO-SFO, which I think departs in the morning from FCO.

Thank you , I will take your advice and look again. I know it would be so much simpler on ITA all the way through, appreciate the info

One last question, If I fly from SFO to Rome then onto Palermo all on the same airline/same ticket where do I go through customs and passport control or do I do that when I arrive in Palermo? And if I do that do I need a minimum time, 1 hour 2hours in Rome to transfer planes?

Passport control (immigration) is in the first port of entry in the European Schengen Zone, therefore that will be in Rome.
The luggage goes through to Palermo and you won’t retrieve it in Rome, therefore customs will be in Palermo, however that will be simply walking through the “nothing to declare” line because there will be no actual customs check unless you are randomly stopped, which never happened to me in over 40 times flying into Italy.
Take whichever flight to PMO the airline website allows you to purchase. If your flight arrives late ITA will simply rebook you on the next available flight, which there are several during the day. One hour could be enough to make the connection.

Thank you exactly the info I needed.