Google is being very open about how long it will support Nexus devices. A support page describes the rules Google has set, and honestly, they’re extremely good. Android version updates are guaranteed for at least two years from launch, and security patches are guaranteed for three years from launch or 18 months after the Google Store stops selling the device, whichever is longer.
Those are some very generous guidelines. At least three years of security patches is a big deal, and we wish every other OEM could guarantee this. And keep in mind that there could be updates and patches after these dates; these are just the minimums.
Google also posted dates for when each device would no longer be guaranteed updates. The list is missing the Nexus 6, but it’s likely an oversight.
It’s definitely awesome that Google is upfront about its support policies. And even when support officially ends, Nexus devices are known to get community support for years. So Nexus owners, you have nothing to worry about.