About every half an hour from early morning to late evening.
Most locals get one mid-morning so they can claim a spot on the beach then make a reservation at their favourite restaurant for a looooong lunch, before having a dip.
Leave your run to the island too late and you'll be at the back of the beach, and hungry (because everyone's made reservations and filled all the tables). After lunch it's a snooze on the beach and a ferry home in the late afternoon.
Same routine for the coastal boats which take you to those secluded beaches/cafes.
(big sigh) take me there NOW! about 10 years ago