how to write javascript for OnKeyPress Event to enter MaxLength of textbox whose textmode propery is "MultiLine" TextBox IN FormView EditeTemplate

<textarea name="Description" id="Description"  onkeyup="CheckLength(this,'250');">Text</textarea>

<script src="https://ajax.googleapis.com/ajax/libs/jquery/1.7.2/jquery.min.js" type="text/javascript"></script>
function CheckLength(obj,  max) {

         if (parseInt(max) > parseInt($(obj).val().length)) {
             
         }
         else {
            alert("Exceded max lentgh");
            
         }
     }