The easiest way to track build failures is to receive the emailed logs and sort them to a folder, so you can maintain a running list of current failures and detect new ones easily. To do this, add an email address to ${branch}/portbuild.conf. You can easily bounce the new ones to maintainers.
After a port appears broken on every build combination multiple times, it is time to mark it BROKEN. Two weeks' notification for the maintainers seems fair.
Note: To avoid build errors with ports that need to be manually fetched, put the distfiles into ~ftp/pub/FreeBSD/distfiles. Access restrictions are in place to make sure that only the build clients can access this directory.