Game updates usually happen when the developers feel that they're playable, fun and ready for release.
For Minecraft: Java Edition there are snapshots showcasing upcoming features before final release. For Minecraft on Android devices, Xbox One and Windows 10, you can try beta versions. If the information hasn't already been posted publicly, we cannot give you a release date.
Here are some links for keeping track of updates:
- Minecraft: Java Edition - Minecraft.net
- Minecraft for Xbox One, Windows 10, mobile platforms (Amazon, Android, Apple, GearVR, Oculus) - Minecraft.net
- Minecraft Legacy Console Editions (Xbox 360, PS3, PS4, PSVita, Nintendo Wii U, 3DS and Switch) - Twitter.com/4JStudios
The Minecraft Wiki also has upcoming Minecraft features listed here. These are a compilation of updates that have been hinted at or announced by Mojang or 4J 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 can find more information how to do that here