Greece in 2-3 days
You won’t get much out of a weekend trip to Greece but it is possible to see Athens in a long weekend or to fly direct to Mykonos for two nights of partying.
In 7-9 days
A typical Athens/Greek Island holiday would involve a one or two night stay in Athens, a flight or ferry to an island (Mykonos and Santorini are good starting points) and then five to seven nights of island-hopping, before flying/cruising back to Athens. If you’ve only got a week or so, it is best to stick to one group of islands
Two weeks +
If you can afford to spend longer in Greece take your time exploring Athens and consider seeing more of the mainland — Delphi, Meteroa and Zagorohoria. You could also pick a few island groups and travel between them.