We have two ferries that come back and forth basically on the ½ hour during summer season. In early summer (June) they run on the hour. What you really need to know is that the first ferry leaves Ammouliani at 7am and the last ferry from Tripiti leaves at 8pm (give or take 5 minutes). In the afternoons they stop for an hour or two between 2-ish and 4-ish.
You can see the ferry schedule here or click this link to Download a copy of the Timetable.