Google last year announced they would slow down to yearly releases because the pace was too quickly, and now that there were more features they didn't need to release so often to get the features out there (or, uhm, to confuse the OEMs and Carriers into making updates all the time? lol)
either way, if you look at 2010-2011, you can see it's going on a yearly release cycle... with a small caveat
2010: roughly every 6 months
January 2010: ?clair (2.1) released with Nexus One
June 2010: Froyo (2.2) released
December 2010: Gingerbread (2.3) released
2011: A year later, ICS is released:
November 2011: ICS (4.0) released.
The caveat I mentioned is that they did get Honeycomb (3.0, 3.1 and 3.2) out in , say, February/May/August (don't remember the exact dates), but that was a completely different branch and not the phone branch.
Here's to hoping now Google will stick to the yearly release cycle. Maybe it'll give OEMs and carriers more breathing room... maybe. I'm an optimist