The airlines are showing connection times of 1h30m in Heathrow and Paris for flights coming from the US to Athens. It seems very tight to me. I called both AirFrance and British Airlines and they both said that the airlines consider this to a "legal" connection. What do you think? Is it enough time?

Check which terminals you are due to use. There are several at both airport. An hour and a half should be sufficient if using the same terminal and the airlines OK it. Travel at Heathrow from terminal 5 to other terminals takes several minutes, and at CDG inter terminals can take best part of an hour. if you are a frequent airport user, fine, but if it is likely to fray your nerves, opt for a later flight.

Thank you. That was my sense.