The Monero Core Team plans for a network upgrade roughly every 6 months. At that time, if you are running a node it must be updated to the most recent version of the Monero software or it will no longer be able to participate in the network.
<sub>1. *Fluffypony gives a great explanation of why mandatory network upgrades are good for Monero.* ([Monero Missives for the Week of 2016-06-20]({{ site.baseurl_root }}/2016/06/20/monero-missive-for-the-week-of-2016-06-20.html))</sub>
