How long should you let your child go online?

AS parents of a 14-year-old boy, we are happy for him to use ever-present media such as the Playstation, iTouch, smartphone and the computer. These give him essential skills: they stimulate his creativity; and are a social platform on which he can communicate, like most of his age group. He still reads books (paper ones), and plays music and sports.
His is the first generation that has grown up with constant screens, but all the hours spent sitting in front of them don’t seem healthy: not for posture and not for social skills.