The goal is to have individual RPM packages available on the mirrors within 72 hours of their release, and normally they are available within 24 hours. Occasionally packages are delayed for various reasons. On rare occasions packages may be built and pushed to the mirrors but not available via yum. (This is because yum-arch has not been run on the master mirror. This may happen when issues with upstream packages are discovered shortly after their release, and if releasing the package would break it's functionality.)
Update Sets (see this FAQ) will have Security Errata released was stated above, while the BugFix and Enhancement errata are actually tested more rigorously and released after the new ISO for the Update Set is produced. The goal for release of a new point release or update set is four to eight weeks after the release by upstream. For more details see the explanation of The CentOS Rebuild and Release Process
During the period between the upstream release of a new minor version and the CentOS release intermediate updates will be published in a Continuous Release (CR) repository.
