Most likely this year. The only ones left are AirPods, MagSafe battery pack I think, assuming iPhone gets it which is very likely given all the leaks.
Magic Trackpad too. Just bought a new one and it came with a usb-c to lightning cable.
Magic Mouse and Magic Keyboard (the desktop version, not the one for iPads) are Lightning as well.
It should happen pretty quickly. No reason not to do it now. Rumors are that Airpods case will be upgraded this fall. That mainly leaves the keyboards, mouse, and trackpads. It’s an easy switch to make.
not long. The 2018 ipad pros already had usb-c. it will just happen.