Yes, there is a limited time to participate in your Challenges. This is clearly visible before starting the Challenge:
It is also visible after activating your Challenge:
<% if (previousArticle) { %>
<% } %>
<% if (previousTitle) { %>
<% } %>
<% if (nextArticle) { %>
<%= previousTitle %>
<% } %> <%= previousArticle.title %>
<% if (nextTitle) { %>
<% } %>