Google offers a page with all the information you'd like to know about updating a Nexus device. However, recently, Google has updated this page with official information about when Nexus devices will cease to receive updates from Google. Usually a Nexus device's lifetime is around 2 years. For two years since their launches, these devices received updates directly from Google before most if not all manufactures can push out updates. (there was the strange case of the Verizon Moto X (2013) which received Kit Kat before the Neuxs 4). Whether it's a full version update (ex: 5.1.1 to 6.0)...
Source: http://www.gsmarena.com/google_posts_details_showing_when_nexus_phones_will_stop_receiving_updates-news-18953.php
