Ubuntu 12.10 (thus named because it came out in October 2012) has just arrived, and 13.04 and 13.10 will come in April and October of 2013. But 14.04 in April 2014 could be the last version released after just a six-month development period.
14.04 is also the next "Long Term Support" or LTS edition. Every two years, Ubuntu is sort of frozen in place with a more stable edition that is guaranteed support for five years. If the change Canonical is considering is adopted, every future edition starting with 14.04 will be an LTS, so the next version after 14.04 would be 16.04 in April 2016.
Ubuntu considers “huge” change that would end traditional release cycle | Ars Technica: