With flights taking around 45 minutes each way, a day trip from Athens to Santorini is technically doable on paper. Whether it's worth doing is a different question.
What a day trip actually looks like
An early-morning flight out, a full day on the island, and a late flight back — realistically giving you six to eight usable hours on Santorini once airport transfers and check-in buffers are factored in. That's enough to see Fira and Oia briefly, but not enough for a beach day, a boat trip, and a relaxed lunch as well.
Our honest take
If Santorini is genuinely not reachable any other way on your trip, a day trip beats not going at all — but it's a rushed, expensive way to see an island whose appeal is largely about slowing down. If you can shift one night from elsewhere in Greece, even a single overnight dramatically improves the experience: you get an actual sunset without a flight to catch immediately after, and a relaxed morning before heading back.
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