Most companies will support their devices with software updates for up to 2 years. Google has actually been pretty good about keeping their devices updated well past the normal EOL cycle of other OEMs. The 2013 Nexus 7, and the Nexus 5 are perfect examples of devices that continue to receive updates far past their prime. Still all good things must come to an end.
A list recently surfaced on Google's support page. It outlines the End Of Life Dates for their devices. The list states that devices will receive android version updates for at least two years from the day they became available in the Play Store. It also says that devices will receive security patches for up to 3 years or 18 months after the last device was sold from the Play Store.
Android updates will not be guaranteed for the following devices after the specified dates.
Nexus 5X : September 2017
Nexus 6P : September 2017
Nexus 9 : October 2016
Nexus 5 : October 2015
Nexus 7 (2013) : July 2015
Nexus 10 : November 2014
We already know that some of those devices have received updates since the listed EOL. These dates are not set in stone, but you should not expect updates after the dates listed. In other words don't purchase these devices in hopes that updates will continue. Of course Nexus devices are easily updated via the developer and Rom community.
via Google
