<p>See the <code>/faq</code> for the rules, specifically</p>
<p><a href="http://chat.stackexchange.com/faq#retention">http://chat.stackexchange.com/faq#retention</a></p>
<blockquote>
<p>Rooms will exist indefinitely, so long as there is at least one person actively talking in the room. A room is considered worth retaining if it has more than 15 messages by at least 2 users.</p>
<p>Rooms not worth retaining which are inactive for 7 days will be deleted. <strong>Rooms worth retaining which are inactive for 14 days will be frozen.</strong> Frozen rooms do not allow any new messages to be sent, and are not shown in the default room list to prevent cluttering the rooms interface.</p>
</blockquote>
<p>Relevant bit bolded.</p>
