Game updates usually happen when the developers feel that they're playable, fun, and ready for release.
Note: If the information hasn't already been posted publicly, we cannot give you a release date.
You can keep track of updates at Minecraft.com.
The Minecraft Wiki also lists upcoming Minecraft features. These are a compilation of updates that have been hinted at or announced by Mojang Studios for the various Minecraft versions. Keep in mind that the listed features have been added by the community and may not be accurate or up to date.
If you have suggestions for Minecraft, you read Where can I Submit Suggestions?