https://support.google.com/playedition/answer/4457705

Nexus devices

Nexus 4, Nexus 5, Nexus 6, Nexus 7, Nexus 9, and Nexus 10 devices receive the latest version of Android directly from Google. Once an update is available, it can take up to two weeks for it to reach your device.

Devices may not receive the latest version of Android if they fall outside of the update window, which is usually around 18 months after a device has been released.