I don't know what that has to do with devices that have already been manufactured and are out in the wild. Phones that are, even today, still being actively sold by carriers.
But, I suppose the Nexus One (21 months), Nexus S (22 months) and the Galaxy Nexus (22 months?) are enough history to know one thing now. Buying a Nexus gives you roughly two years of support. That's better than Google's committed 18 months, I suppose.
Of course, the decentralized nature of the OS, especially for newer devices like the GNex and N4, make this much less of an issue than it is on iOS.
And if they keep up their Nexus releases every year with the awesome hardware baseline they started with the N4 and kept going to the N5, I don't think anyone here would really complain about having to shell out $350 a year for the latest and greatest Nexus phone.
Nexus One - 5 January 2010; Android 2.3.6 released 2 September 2011
Nexus S - 16 December 2010; Android 4.1.2 released 9 October 2012
Galaxy Nexus - 17 November 2011; Android 4.3 released 24 July 2013
Nexus 4 - 13 November 2012; Android 4.4 released 31 October 2013
Nexus 5 - 31 October 2013; Android 4.4 released 31 October 2013
So..
Nexus One = 1 year, 7 months, 28 days
Nexus S = 1 year, 9 months, 23 days
Galaxy Nexus = 1 year, 8 months, 7 days
Nexus 4 = 11 months, 18 days (so far)
Nexus 5 = 1 day (so far)