We can only judge by devices that are out of support.
Galaxy Nexus: released in November 2011, last release: 4.3 in July 2013. Update Window: 1 year and 8 months.
Nexus 4: released in November 2012, last release: 5.1.1 in May 2015. Update window: 2 years and 6 months.
Nexus 5: released in October 2013, last release: 6.x (?), probably April/May 2016. Update window: probably 2 years and 6 months.
(Granted, the Nexus 4 received security updates for a longer period.)
They have recently announced a better policy:
Nexus devices will continue to receive major updates for at least two years and security patches for the longer of three years from initial availability or 18 months from last sale of the device via the Google Store.
Galaxy Nexus: released in November 2011, last release: 4.3 in July 2013. Update Window: 1 year and 8 months.
Nexus 4: released in November 2012, last release: 5.1.1 in May 2015. Update window: 2 years and 6 months.
Nexus 5: released in October 2013, last release: 6.x (?), probably April/May 2016. Update window: probably 2 years and 6 months.
(Granted, the Nexus 4 received security updates for a longer period.)
They have recently announced a better policy:
Nexus devices will continue to receive major updates for at least two years and security patches for the longer of three years from initial availability or 18 months from last sale of the device via the Google Store.
Source: http://officialandroid.blogspot.de/2015/08/an-update-to-nexu...
But 3-4 years is definitely false. Up to now ~2-2.5 years, in the future 3 years.