Update: The Apple Watch Series 2 launches in stores on Friday September 16, 2016.
The Apple Watch 2 will launch in 2016. Look here for updates as more details surface.
For reference, the original Apple Watch shipped on April 24, 2015. Pre-orders of the device opened on April 10, 2015. If Apple retains the annual hardware updates that iPhone users have become accustomed to, the next Apple Watch should launch in spring 2016.