I don't know of any official set policy beyond 18 months including OS updates. Google has committed to that.
BUT...they have been supporting most Nexus devices quite a bit longer. The Nexus 5 and Nexus 7 2013 are about 2 1/2 years old and got Marshmallow 6.0 which pretty much assured updates to 6.0 will apply for another year, making about 3 1/2 years of support. The Nexus 6 will likely get similar support. That's good support. I'm happy with it!
Now, if you don't own Nexus devices and support time is short, take it up with the manufacturer, not Google.