How long is each Joomla! version supported?
Each major release of Joomla is supported for about 3 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 about 3 years (January 2011 until spring 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 spring of 2014. So the 2.x series has a little over three years of support and, overall, it was a smooth experience for users and extension developers.
The 3.x series will have about 3 years of support as well. 3.0 to 3.1 to 3.5 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.