openproject/app/assets/javascripts/settings.js.erb

<%#-- copyright
OpenProject is a project management system.

Copyright (C) 2012-2013 the OpenProject Team

This program is free software; you can redistribute it and/or
modify it under the terms of the GNU General Public License version 3.

See doc/COPYRIGHT.rdoc for more details.

++#%>

jQuery(document).ready(function($) {
  $('#settings_session_ttl_enabled').on('change', function(){
    $('#settings_session_ttl_container').toggle($(this).is(':checked'));
  }).trigger('change');
});