Changing cache lifetime for wp_feed_cache

Hi guys, this is the code I’m using to extract info from an external feed:

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I’d like to change the caching lifetime (I think that default is set to 10 hours).. do I need to put this in functions.php or I can also add it right after the <?php tag..?

add_filter(‘wp_feed_cache_transient_lifetime’, create_function(‘$a’, ‘return 300;’));

Thanks in advance! ??