Koufonisia is the small, sandy star of the Small Cyclades, south-east of Naxos, and getting home from it is a proper journey: around 112 nautical miles, or about 208 km, to Piraeus. There is no airport on the island, so the ferry is the only way out, either direct to Athens or via a bigger island such as Naxos.
You leave from the harbour in Chora, the main village of Ano Koufonisi, the inhabited island of the group, with cafés, restaurants and small hotels right by the quay. The ferry arrives in Piraeus, Athens' main port, where metro Line 1 and the tram, the suburban railway and buses connect you to the city and the airport.
Two companies run the direct route, and they are very different. Blue Star Ferries keeps a year-round service on three or four days a week, on large conventional ships that stop at several islands on the way and take most of a day, roughly 7 hr 15 min to 9 hr 5 min. Seajets adds high-speed boats in the warmer months that do the trip in a little over four hours on direct runs, though some of its routings take over 8 hours. The fast boat costs more: from €79.70 in Ferryhopper's table, against €43 for Blue Star.
Check your sailing days first. With no daily Blue Star boat and Seajets only in season, the day you leave Koufonisia may be fixed by the timetable rather than your plans, and if you have a flight from Athens it is wise to arrive the day before.