Reliable Android updates are a key selling point of Google's Nexus line of phones. Even Nexuses have an expiration date, though. Now, Google has a published list of when you can stop expecting updates for your Nexus phone or tablet. The list can be found here, on one of Google's support pages under "How long your Nexus will get updates". Currently the Nexus 5X and Nexus 6P are the safest, with promised Android version updates lasting at least through September 2017. Most other Nexus devices are either past their expiration date already or, in the case of the Nexus 6 and 9, will expire in October of 2016.
Google specifically notes that past these dates there's no guarantee of an update. It's entirely possible that Google may decide to give a little more life to certain devices at their discretion, but you probably shouldn't count on it. Also, these dates only apply to Android version updates. Nexus devices will continue to get monthly security updates for "the longer of 3 years from availability or 18 months from when the Google Store last sold the device".