I understand that there is a long line at the airport in Santorini for flights back to Athens. We have a 9:00 p.m. flight in May. How early do we need to arrive at the airport?
Santorini is a small airport. I flew out a few days after Easter and I was able to check in and get through security in about 15 minutes. My flight was to Athens.
Athens on the other hand was very crowded. Give yourself time.
Perhaps someone with more recent experience can chime in.
The day before your departure you can consult the flights departing from Santorini here:
https://www.jtr-airport.gr/en/flight-list
This will give you an idea of how busy the airport will be.
With Aegean/Olympic or Sky Express you can check in online 48 hours before departure and get your boarding pass on your smartphone.
Our experience at Santorini airport was that the bus to town was often late. Since we only used the airport going INTO Santorini, inconvenient but not a disaster. (We take the ferry to Naxos or Paros leaving the island).