Joomla Version 2.5.0 is the second release made within the new six-month release cycle that started with the delivery of Joomla 1.6 in January 2011. Version 2.5 is a long-term-support (LTS) release that will be supported for at least 18 months.
Joomla Version 1.7 reaches end of life on 24 February 2012. All users of version 1.7 are advised to update to version 2.5.0 before that time.
The following list of key dates in the development cycle should prove useful.
Long Term Support Release: Joomla 2.5
General Availability Date: January 24, 2012
End of Long Term Support: December, 2013
This is the culmination of the 1.6/1.7/2.5 series
Long Term Support Release: Joomla! 1.5.25
End of Support: April 2012. Major security fixes will be done until release of 3.0 in September 2012.
Standard Support Release: Joomla 1.7.4
End of Support: February 24 2012
Upcoming Major Release: Joomla! 3.0
Current status: Planning
Expected General Availability Date: September 19, 2012