In wpForo, you can use the wpforo_recent_posts_limit filter hook to change the time period of displaying recent posts. For example, you can display recent posts made during the last 365 days:

add_filter( 'wpforo_recent_posts_limit', 'wpforo_custom_recent_posts_limit' );
function wpforo_custom_recent_posts_limit( $limit ) {
    return 365;
}

You can change the 365 to any value you want.
You can add this code in functions.php file of your current active WordPress theme or use a plugin which allow you to add custom PHP code snippets like Code Snippets.

After added the code, do not forget to delete wpForo cache in wpForo > Overview admin page, scroll down, find and click the [Delete All Caches] button. This code is only designed for Recent Posts page, it doesn't work for widgets.