Google has provided significant and frequent incremental updates for their Android mobile platform.
1.0 October 2008
1.5 April 2009
1.6 September 2009
2.0 November 2009
2.1 February 2010
Google didn’t provide a release date for 2.2 (Froyo). But, based on this information about the release that follows 2.2, I wouldn’t be surprised if Android OS 2.2’s beta period is very short.
My guess is that Froyo will become available for handsets around the same time as iPhone OS 4.0: Late June or Early July.