Apple does not have a fixed release cycle for the iPad, as of May 2015. However, Apple has introduced new models once per year since the debut of the first iPad.
The original iPad debuted in April 2010. The iPad 2 was released a little less than a year later, and the third-generation iPad was released almost exactly a year after that. Around 18 months later, Apple released the fourth generation iPad and the iPad Air, followed less than a year later by the iPad Air 2. Apple has not announced a future release date for the next generation iPad.