How to get its meta_value of a specific meta_key within wp_usermeta

@Cristián Lávaque Thank you for the hints 😉

Actually I managed to resolve the issue blocking this function (show the hidden comment above). In case this would help anyone here is the final working code:

function check_post_limit() {
    if ( current_user_can( 'edit_posts' ) ) {
        global $userdata;
        global $wpdb;
        $s2member_last_payment_time = get_user_meta( get_current_user_id(), 
            'wp_s2member_last_payment_time', true );
        $postintervall = date( 'Y-m-d H:i:s', $s2member_last_payment_time );
        $item_count = $wpdb->get_var( "SELECT count(*) FROM $wpdb->posts 
            WHERE post_status="publish" AND post_author = $userdata->ID 
            AND post_date > '$postintervall'" );
        if ( ( $item_count >= 2 ) && ( current_user_is( "s2member_level1" ) ) ) { 
            wp_die( 'Do not create too much posts.', 'check-limit' ); 
        }
        return;   
    }
}