They leave Fethiye on the hour and half-hour. So, depending on what part of Ovacik you're referring to, they're usually going along the mainroad 20-30 minutes later. It's roughly 35-45 minutes from Fethiye to the centre of Hisaronu.
The last dolmus out of Kaya that you can generally rely on leaves the centre of the village at 21.00, but sometimes there are also a couple of later ones.