According to the Joomla Roadmap, version 3.5, the next long term support (LTS) release, will be available in November, 2014. Support for version 2.5 is scheduled to stop at the end of 2014. So, ideally you will want to be on Joomla 3 by the end of 2014 to avoid the unnecessary risks of being on an unsupported version. What risks you ask? Hackers and malware causing problems on your website due to security holes that will never be fixed. Keeping your Joomla software and extensions current is the best defense against this.