The most recent one on slackware-security I received was march 22th. Generally speaking, the ChangeLog.txt for slackware-current as well as the /patches directories of the stable releases is the first place to see security updates. If you don't need or don't want to actively follow these ChangeLogs, then it is good to have a subscription to slackware-security.
In my logs, the last slackware-announce I have is when Slackware 10.0 was released, and before that, the announcement of Slackware 8.1 :-)
So don't expect too much from that mailing list subscription.