The Release Engineering Team < re@FreeBSD.org> releases a new version of FreeBSD about every four months, on average. Release dates are announced well in advance, so that the people working on the system know when their projects need to be finished and tested. A testing period precedes each release, in order to ensure that the addition of new features does not compromise the stability of the release. Many users regard this caution as one of the best things about FreeBSD, although waiting for all the latest goodies to reach -STABLE can be a little frustrating.
More information on the release engineering process (including a schedule of upcoming releases) can be found on the release engineering pages on the FreeBSD Web site.
For people who need or want a little more excitement, binary snapshots are made every day as discussed above.