When is the next minecraft update being released?

That information has not been released yet.

It is most likely 1.8.
If you want to see some upcoming features, look here.
http://minecraft.gamepedia.com/...g_features

Minecraft's 1.7.3 Pre Release version is already out. You can checkout the article below:
http://hostthegame.com/minecraft-1-7-3-changelogs-updates-bug-fixes/