A Blue Star ferry from Santorini to Athens takes about about 10 hours. The first few hours are fine while you are calling at various islands on the way and have their buildings and colour schemes to admire.
This is a church on Ios.
Each stops sees new passengers come on board as others leave. Nothing disciplined about boarding - it's every person for themselves. Surprisingly there are very few problems amongst the passengers. I am sure our OSH would find a reason to stop a sailing!
The journey gets even better when the sun begins to set. This makes the the hour or so before sunset a paradise for photographers.
The last hour to Athens is a bit long - especially when that hour finishes at about 12:45 am!
At that time of night a taxi is a good option even if it costs 26 euros to get from the port to the centre of town. There are buses and trains but that would have made it an even later night!