Alan,

Try the following code:

<script language="JavaScript" type="text/javascript" >

var editor = document.getElementById("editor1_editBox");
editor.onkeypress = maxlength;

function maxlength()
{
    if (editor.innerText.length >= 500)
    {
        //do something here
        alert("Max length is 500");
    }
}

</script>

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