I ve used

<h:form id="form">
 <script>
 function checkMessages(){
 if (document.getElementById('form:messages')==null){
 document.getElementById('form:button').disabled=false;
 }
 else{
 document.getElementById('form:button').disabled=true;
 }

 }
 </script>
 <h:inputText value="#{bean.property1}" required="true">
 <f:validateLength minimum="3" maximum="5"/>
 <a4j:support event="onkeyup" bypassUpdates="true" ajaxSingle="true" oncomplete="checkMessages();"/>
 </h:inputText>
 <a4j:outputPanel ajaxRendered="true">
 <h:messages id="messages"/>
 </a4j:outputPanel>
 <a4j:commandButton value="Press me!" id="button"/>
 </h:form>

For such functionality.. Unfortunatelly I'm failed to use rich:messages - because to be ajax updatable they always generate span with messages id ://