How long is each Joomla! version supported?
Each major release of Joomla is supported for over 4 years and includes one-click upgrades. For example, the 2.x series (which included 1.6 and 1.7 due to naming conflicts) has a support life of over 4 years (January 2011 until December 31st, 2014). 1.6 to 1.7 was a one-click upgrade. 1.7 to 2.5 was a one click upgrade. 2.5 will be supported until December 31st of 2014. So the 2.x series has over four years of support and, overall, it was a smooth experience for users and extension developers.
The 3.x series will have over 4 years of support as well. This of course is taking into account all the STS versions, 3.0 to 3.1 then 3.2 which will only have 7 months of support each, followed by last version, 3.5. The final version of the 3.x series will be Joomla! 3.5, an LTS version, planned for release in March of 2014 with support until the end of 2016. All of the Joomla! 3.x versions will be one-click upgrades and should, overall, be smooth transitions for users and developers as the 2.x series was.
Because Joomla's new timed-release cycle makes moving from major version to major version (e.g. 2.x to 3.x) relatively simple, users get the added benefit of compounded support, where they can move from major version to major version to get continued and uninterrupted support.