Looking for advice. I have an 11:35am flight scheduled from ATH back to the United States. I need to book a flight from Santorini to ATH that same morning. The flight from Santorini arrives in ATH at 9:10am (Ryan Air). That would leave roughly two and a half hours from arriving in ATH to get our luggage, then get checked in for our 11:35am flight back to the US. Will two and one half hours be enough time? Are Ryan Air flights usually on time?

I have no idea if Ryan air is reliable.
I can say that 2.5 hours is probably lots of time at Athens airport which is small and only one terminal so pretty efficient.

I can also say we were on a flight a couple of years ago with Americans, Brits, French and South Africans who planned to fly to Athens then catch their return flight home same day. The flight was delayed and they all missed their connections.

Conventional wisdom from experience Greek travelers say, get back to Athens the day before your return flight. Stuff can happen. Think about it.

"Are Ryan Air flights usually on time?" I presume you are looking at FR1233?
You can find out this kind of information readily online
flightaware.com/live/flight/RYR1233/history/20160215/0620Z/LGSR/LGAV

I think you are approaching the issue as either you can make it or cannot make it. While AFTER the event, it is either of the the two outcomes, at this point, there is only a probability. Suppose the probability is favorable to you now, but if on that day, the flight is hopelessly late or cancelled, what are you going to do? If you can tolerate the outcome, it is a risk worth taking based on the favorable probability. You can further improve your odd by not checking your luggage.

It's always wise to return to Athens the day before your flight home. If possible save all your time on the mainland for the end of your holiday so you aren't wasting time or backtracking, and so you aren't stressing about missing that flight back home.

Do you have travel insurance that will cover the costs of a missed flight? Ryan Air will not help you if they are late and cause you to miss your connection.

It is a risky game that you are intending to play.

Albeit your Ryanair flight usually seems to be on time there is always a chance that the flight is late or cancelled altogether. Sometimes, a technical problem occurs, and, unlike the large carriers, Ryan would not have a substitute aircraft. Or a pilot reports sick (we have had that once on a flight). Or whatever.

If something happens and you miss your transatlantic flight your ticket is gone (unless you have one of those expensive flexible tickets). You may have to buy a first class ticket to get a seat on the next day's flight home. And neither Ryanair nor travel insurance will cover the extra cost.

I always go to the city of my transatlantic departure at least the day before.

Thank you all for the suggestions! For piece of mind, I think we will return to Athens the day before to ensure that we do not miss our flight back to the states. I appreciate all of your help!