Mark Shuttleworth has written a good blog post detailing about future Ubuntu release schedule and the length of support for a particular release, whether its a standard release or an LTS (Long Term Support) release.
In his post, he also explains about the point release concept for the LTS, which each Long Term support installation will receive continuous updates up until the next release of LTS. Which is due to be released in two years time. This point release will allow fresh LTS installation to receives new patches and updates without going through the hassle downloading them through the internet.
Standard releases will continue to receive patches and updates for one and a half-year and Server LTS release will continue to receive updates for 5 years.
For more information read Mark Shuttleworth post, The Art of Release