<script type="text/javascript">document.getElementById('title').onkeyup = function(){document.getElementById('warning').innerHTML = this.value.length > 80 ? this.value.length + ' / 80 maximum characters' : '';return true;};</script>

Requires setting an "id" attribute on the title input field and then adding another element underneath of it with a "warning" id (<br><span id="warning"></span> works nicely).