Fetch_Feed : How Can I change the Cache Time

I wanted to display an External RSS from mybb forum to my wordpress blog. I found out the function fetch feed (http://codex.wordpress.org/Function_Reference/fetch_feed). According to the codex page this function uses SimplePie and FeedCache functionality to retrieve, parse and cache an external feed. My blog is hosted on a shared server and I am worried about server overload. So I wanted to find out when the cache expires. The SimplePie documentation tells that cache expires after every 60 mins. Now the extrenal feed I want to display does not change more than once or twice a day. So how could I change these 60 mins to 180 mins.

Secondly, the fetch_feed page describes a method to display 5 items from a feed. But mybb forum’s syndication could be queried for number of items like this http://www.example.com/forum/syndication.php?limit=5 so I basically don’t need to check the items. So how do I change the code so that it doesn’t check the number of items in the feed. Below is the Fetch_Feed example from codex.

<h2><?php _e('Recent news from Some-Other Blog:'); ?></h2>
<?php // Get RSS Feed(s)
include_once(ABSPATH . WPINC . '/feed.php');

// Get a SimplePie feed object from the specified feed source.
$rss = fetch_feed('http://example.com/rss/feed/goes/here');

// Figure out how many total items there are, but limit it to 5.
$maxitems = $rss->get_item_quantity(5); 

// Build an array of all the items, starting with element 0 (first element).
$rss_items = $rss->get_items(0, $maxitems);
?>

<ul>
    <?php if ($maxitems == 0) echo '<li>No items.</li>';
    else
    // Loop through each feed item and display each item as a hyperlink.
    foreach ( $rss_items as $item ) : ?>
    <li>
        <a href='<?php echo $item->get_permalink(); ?>'
        title='<?php echo 'Posted '.$item->get_date('j F Y | g:i a'); ?>'>
        <?php echo $item->get_title(); ?></a>
    </li>
    <?php endforeach; ?>
</ul>