According to Google's announcement, the personal voice assistant that launched on thephone will slowly make its way to all devices in the US with English set as the device language.
Yes, except the exact timing of when your device will actually receive the update is a giant mystery.
While I can't tell you exactly when your phone will start using Google Assistant, you can take a few steps to ensure it will happen sooner than later.
Join the Google Play Services beta by opening this link and clicking on "Become a Tester" using the same Google account linked to your Android device.
Wait a few minutes, then open the Play Store and go to the My Apps section. In there should should be an update listed for Play Services. You may have to open the Beta tab and manually start the Play Services update. While you're in the Play Store checking for updates, make sure the Google app itself doesn't have any pending updates.
With all of your apps updated, now you wait. To check for Assistant on your Android device, hold the home button down for a few seconds.
One of these days, eventually, maybe, possibly... you will see a prompt asking you to begin the setup process for Google Assistant instead of Google Now opening.
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