There was some discussion a while back to maybe make MariaDB Server follow the Ubuntu release model, i.e. having a Long Term Release (LTS) and then having a few regular fast releases with a shorter support cycle.
However its good to note that the decision now going forward is to support each and every GA release for a period of five (5) years. However, regular releases will only happen for the latest three (3) GA releases, so at this moment, you are getting updates for MariaDB Server 5.5/10.0/10.1.
Practically, we’ve not seen an update for 5.1/5.2/5.3 since 30 Jan 2013 at the time of this writing. And its clear MariaDB Server 5.5 will have an extended support policy, as it ships in Red Hat Enterprise Linux 7.
At this time it’s worth noting that for MySQL 5.5, premier support ends December 2015, while there is extended support till December 2018.