How long is the action implementation period?

According to Guidelines for Grant Applicants 2.1.1.2.1.

The planned duration of an action (implementation period) may not exceed 24 months. In duly justified cases the action implementation period can be prolonged by decision of the Joint Managing Authority during its implementation. In any case the implementation period can be no longer than until the 31st of December 2014. The shortest action implementation period is not defined.