1 hr 35 min or 8 hr layover in FCO?

Check the on-time performance of the flight from Chicago over time - a late arrival would be the determining factor in making your connection. If they're selling a ticket with a 90 minute layover, they're fairly confident that it's possible.

I believe you will have to go through immigration at FCO - when I arrived in Rome back in November it took very little time using the E- gates, but that was leaving the airport, not connecting.

We were on flights from both Los Angeles and Miami - both arrived early. But YMMV.

Look also at whether other flights from FCO to Athens are available that day. You may be able to book a longer layover if that would make you more comfortable or at the very least you can see whether you would have to take a flight the next day if you miss your connecting flight.

If your flight to Santorini is on a separate ticket, buy a class of ticket that allows for ticket changes. You have plenty of time for unconnected flight, assuming you arrive as scheduled. I am thinking if you miss your flight in Rome to Athens you will want to contain the damage.

Thank you both! I did end up looking into the history of the flight, and it seems to generally arrive on time.

There is also another flight option through the same airline for the same price, but that would have an 8 hr 15 min layover.

Would the 8 hr layover be better just to be safe? It seems like a waste of time and I'd rather arrive in Greece earlier, but I can't decide if 1 hr 35 minutes (it appears we'd need to change terminals) is too close for comfort.

Are you that risk averse that you would endure an 8 hour layover after a trans-Atlantic flight rather than take the risk on a 1.5 hour connection that the airline is selling you? Lots of airports have connections that are less than an hour (e.g. Munich). I wouldn’t think twice about the 1.5 hr connection. Just do a little research ahead of time to figure out back up options

I would also take the 1.5 hour connection. 8 hours is too long! I am not that risk adverse.

I faced the same situation at CDG. Took the longer connection (but four hours not eight!). Would have made the original connection.

Last year I had a 65 minute connection in FCO, between landing from Canada and going onward to Sicily.
I used the loo on the plane before landing, got off my first flight as quickly as I could, followed all the signs carefully and kept checking for my next gate at nearly every sign board I passed.
I walked quickly and I did have a carryon bag only.
I was at my gate in plenty of time and I have knees that don’t always cooperate.
If you are all on one ticket booking you will be fine.

We just arrived in Greece yesterday. As long as it is not an extremely busy time I would think you could make it especially without checked bags

I see that ITA sells an ORD-FCO-ATH ticket with a 1 hr stop at FCO, so your 1hr 35m transfer shouldn't be a problem.

If you are flying both ways from Athens to Santorini and back, I would buy 2 one tickets rather than a r/t ticket. That way if something goes sideways and you do not get to Athens on time you have only one leg to re-book and thus minimize your loss.

As long as you buy the single ticket itinerary, the 95 minute layover should be fine.