From Wikipedia:
Lightroom 1 beta: 29 Jan 2007, Release Feb 2007 (I think)
Lightroom 2 beta: April 2008, Release July 2008
Lightroom 3 beta: October 2009, Release June 2010
Lightroom 4 beta: Jan 2012, Release March 2012
Lightroom 5 beta: April 2013, Release June 2013
This ties with my credit card records for the release versions. In other words: the interval between major releases varies from 15 months to nearly 2 years. And the beta period varies from 2 months to 8 months.