Is 1.5 hr Frankfort airport layover enough for flight back to the US?

On a single ticket, you WILL go through passport control, but your luggage will be checked through to home.
I would take a chance on 1.5hr at Frankfurt, but if it would cause you considerable stress or financial setback to miss the flight, then you would probably want to have at least 2.
I have run through Frankfurt airport on more than one occasion, but I always made it (albeit sometimes with only seconds to spare, and I swore last time I'd avoid Frankfurt.) If on arrival you are transferred on a bus, it can feel like you are stuck on that bus forever. No one from Lufthansa facilitates getting you to the gate (which would be a big help since they know better than anyone how *&^@@$ed up Frankfurt is.)
The good news is they do have to get you home if you miss the flight because they layover is too short, so I'd probably risk it verses having to be in the airport longer.

Thank you for the information painting an honest picture of the "risk". That's exactly what I need.

I've done 90 min or less several times at FRA. Not fun. But always made it. Worst case they have to put you on the next flight and FRA is busy airport, so that might not be a long wait. It's something you can research in advance for peace of mind.

Over the past two years, I have flown through Frankfurt and have tried to book at least 2 hour transit time. Over the past year, I have flown through Frankfurt with 1.5 hour layovers and missed my flight once due to delays in arrival to Frankfurt. The other issue with Frankfurt is sometimes you are dropped off on the tarmac and have to take a bus which can introduce delays. The other flight last year that was 1.5 transfer, my flight from Rome was delayed. I was able to run to catch the plane, but my luggage took a later flight.

If you decide to take 1.5 hour layover, just know when the next flight is, so you can go to customer service to secure that next flight.

Thank you everyone. I will not book the flight with 1.5 hr layover. Too much stress and the memory of having to stand in line for 4.5 hrs to get a new flight at Frankfurt airport due to missed connections three years ago still hurts. They were so short on staffing at the time right after covid. It was excruciating.

I think your decision to forego a 1.5 hour layover is the right one. I traveled often to Europe for work (retired now) and Frankfurt was the most convenient direct flight back to the US for me and my colleagues. I learned to always fly in to Frankfurt the night before my morning flight back to the US. Tight connections and flight delays often caused colleagues to miss the flight home.

I guess I have been lucky. Never missed a connection out of no less than 20 in 20 years. Quite a few 90 min or less. BUT, if the cost is more or less the same and there is something less than 3.5 hours layover that I can get, yes, that would be my first choice. Would I pay 25% more for that ... no.

I too would be happy to chance a 1.5 hour layover, but FRA is becoming more & more busy, so a longer layover is safer.

Too much stress and the memory of having to stand in line for 4.5 hrs
to get a new flight at Frankfurt airport due to missed connections
three years ago still hurts.

The first thing I would do in the case of a missed flight is to check the app to see if I've been automatically booked onto the next flight, AND check my seat assignment. If you get stuck in FRA, or for that matter in any German airport, AND you're flying on a US carrier, you can also find a cozy corner & call the US telephone number for your carrier (I have done this a few times with United), explain the missed flight and they will book you on the next flight, and get you a seat. Maybe this suggestion would work with European carriers as well on the phone, but I don't have personal experience. A great reason to have Viber or another very cheap alternative to make international calls, but that's a different subject.

Frankfurt Airport is kind of a large confusing dinosaur, indeed. And the risk is often
increased when the smaller planes (i.e, your inbound Athens flight) parks at a gate on
the tarmac and you have to take the bus into the terminal to start making your connection.

Having said that, the e-gates would most likely help reduce the passport check wait time.
Assuming you are on LH/UA, you most likely just have to make your way from the A level
to the Z-Level, but it can still be a long walk.

If you have lounge access, then derisking your connection also comes with the chance
to relax and even take a shower before the long flight home.