QGIS 2.18 is the current LTS version, as 2.14 is now expiring. Unless you feel comfortable compiling from source or adding alternate repositories, your best bet is to stick with your distro's release. QGIS 2.14 is quite usable, but that depends more on your requirements for functionality.
In your case, looking at the Debian/Ubuntu section on qgis.org, we see that "Default Debian and Ubuntu software repositories often hold older versions of QGIS. [...] To have newer versions, you have to add alternative software repositories, by adding one of the deb-lines below to your /etc/apt/sources.list file."
Here you can see your options for installation, including adding alternate package sources. As long as you configure your repos correctly to account for all dependencies and have no package conflicts, you will be fine installing 2.18 on Ubuntu 16.04.
A simple tutorial to get you started can be found here. If you want an answer on when Ubuntu will release the new LTS, you'll have to ask the package maintainers. According to the QGIS governance page, that would be:
* Jürgen E. Fischer <jef[at]norbit.de>
* Tim Sutton <tim[at]linfiniti.com>
* Jachym Cepicky <jachym.cepicky[at]gmail.com>