User Post Type – force a sync every 8 hours

<?php

add_action( 'mysite_sync_users', 'mysite_sync_users_callback' );

function mysite_sync_users_callback() {
    UPT()->sync->run_sync();
}

if ( ! wp_next_scheduled( 'mysite_sync_users' ) ) {
    wp_schedule_single_event( time() + 28800, 'mysite_sync_users' ); //  28800 seconds = every 8 hours
}