As a general principle, the FreeBSD core team only release a new version of FreeBSD when they believe that there are sufficient new features and/or bug fixes to justify one, and are satisfied that the changes made have settled down sufficiently to avoid compromising the stability of the release. Many users regard this caution as one of the best things about FreeBSD, although it can be a little frustrating when waiting for all the latest goodies to become available...
Releases are made about every 4 months on average.
For people needing (or wanting) a little more excitement, there are SNAPs released more frequently, particularly during the month or so leading up to a release.