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Check for element to appears in the DOM and do stuff and stop interval

<script>
jQuery(function($){
myVar = setInterval(function() {
if ($('.x-class').length) {
    $('.contact-form-home .et-pb-contact-message li:last-child:not(.appearedx)')
        .addClass('appearedx')
        .each(function() {
            $(".appearedx").text('Math Problem');
        });
    clearInterval(myVar);
    }
}, 100)
});
</script>

myVar = setInterval("javascript function", milliseconds);
Then you will be able to stop the execution by calling the clearInterval() method.

clearInterval(myVar);

code type: 
js