It'll probably be a few years, and it'll probably start with the front ports and hubs before the rear ports start to get changed over. USB's got 20 years of devices to support, and a lot of people would rather not have to get all new cables for their existing devices. Though I do foresee that things like mice and keyboards will ship with A<->C dongles for a few years, much like the USB<->PS/2 adapters of 15 years ago.
Yes, the Mac had USB about 5 years before it became popular on the PC. However, you aren't clear as to why they can't simply throw in a couple small USB-C connectors now. No one has to buy new cables. You'll still have all those legacy ports.
