Hi
I did this trip with Ploppy and Marge last year, great day out. The boat journey was about 1.5 hours each way if my memory serves me correct. But the views as you look back towards the south coast of crete are worth every minute.
If you go, don't be disappointed when you first get there as the harbour is very unattractive. The local bus puts on a little excursion around the island for a few euros and then takes you to an excellent beach where you can eat, sleep, drink, swim, whatever takes your fancy (I think we did all four).
Then Lefteris takes you to take a look at the south coast, won't spoil any surprises but it's amazing.
Try and go it's a good day out but be prepared for disappointment, if Lefteris does not have enough people he can not go.
Ali