To be honest, unless you speak Turkish, you'll probably find it pretty hard to communicate exactly what you want with a Turkish hairdresser, especially if you have any complex needs. If you leave yourself in a hairdresser's hands, you're likely to have a bit more taken off than you expected, or get one of the famous "Apache" cuts you can see on legions of young men.
Most of my visitors have instead opted to get a shave. Every hairdresser offers them, and a good straight razor shave is an absolute blessing. If you find a place near where you're staying, they are generally cheap enough (less than 7tl normally) that you can leave your shaving kit at home and treat yourself a few times during your vacation.