In a recent webinar on implementation of Common Core State Standards, OSPI shared the timeline for implementation of the Next Generation Science Standards. I would assume this is probably a similar timeline for other states, or at least the lead state partners who already support the NGSS effort.
1. NGSS Final Draft Released Spring 2013
2. NGSS Adopted by WA State January/February 2014
3. NGSS Assessed 2016/17 (but more likely 2017/18)