See the COMMENTS section of...
- Windows 10 1607 is now a Current Branch for Business (CBB) release
https://blogs.technet.microsoft.com/windowsitpro/2016/11/29/windows-10-1607-is-now-a-current-branch-for-business-cbb-release/
<QP>
As we described in the Windows as a service docs at http://aka.ms/waas, we service and support two CBB releases at all times, so the end of servicing for a particular release is dependent on the timing of the N+2 release. It will be at least 18 months, but can be longer
when releases are more than six months apart.
Because there are two newer CBB releases, Windows 10 1507 [AKA Win10 RTW] will not be serviced much longer – as I mentioned in a previous reply, the 60-day countdown (the grace period described in the Windows as a service docs) will start in January, meaning
the last updates for Windows 10 1507 will be in March. [emphasis mine]
For Windows 10 1511, the end of servicing will be about six months after the CBB declaration for the next Windows 10 feature update (the Creators Update, due out early next year). So
Windows 10 1511 will be serviced at least through the middle of 2017, possibly longer depending on the actual release date of the next feature update. [emphasis mine]
</QP>
Per the above, the 60-day countdown for Win10 RTW begins in January 2017.