I don't know if there is a specific event for this, however what you can do is use jQuery to detect a keyup
event on the editor.
Firstly, inspect the edit element and get the class or ID. Then your jQuery could be something like this:
jQuery(document).ready_function($){
$('textarea').on('keyup', function() {
// do something here
});
});
Update:
You can use the Joomla API to inject this jQuery, like so:
$doc = JFactory::getDocument();
$doc->addScriptDeclaration("
jQuery(document).ready_function($){
$('textarea').on('keyup', function() {
// do something here
});
});
");