A couple of nights (3 days) will be sufficient to see the main sites. Then, we strongly advise you to visit places and overnight out of Athens instead of touring back and forth from Athens!
Even one full day (2 nights) can be enough of a stay in Athens!
The magic of Greece is in the countryside and the mainland but do include an island or two!
☼ Do check out our FAQs Not 4 Dummies section to see all our FAQs. It contains a wealth of knowledge for the first time but also the experienced visitor of Greece.