Here is the official document of the Formidable form. https://formidableforms.com/knowledgebase/javascript-examples/javascript-after-form-submit/

You can try something below:

<script type="text/javascript">
    jQuery(document).ready(function($){
        $(".frm_message").delay(10000).fadeOut(1000); // change 5000 to number of seconds in milliseconds  
    });
</script>

Don't know about that plugin but a few lines of JS should do the trick.

Let's say the element that wraps your success message has a class called notice-container and the form element has the id form21

document.addEventListener('DOMContentLoaded', function () {
    let container = document.querySelector('.notice-container'),
        form = document.querySelector('#form21');

    if (form != null) {
        form.addEventListener('submit', function () {       
            setTimeout(function(){
                container = document.querySelector('.notice-container');
                if (container == null) return; // abort if element isn't available

                container.style.display = 'none';
            }, 10000);
        });
    }
});

Place this code anywhere on your page and you're good to go.

P.S: Don't forget to replace the notice-container and form21 values accordingly to your form