Maybe this will work?

snippet.php

<?php
// Download this snippet as a plugin and more at: https://wordbits.io/snippet/extend-public-post-preview/
// Created by: Liam Dempsey
?>

<?php
add_filter( 'ppp_nonce_life', 'my_nonce_life' );
function my_nonce_life() {
    return 60 * 60 * 24 * 5; // 5 days
}