You are actually multiplying 300*5 in your code giving you 1500 seconds or 25 minutes. I'd say the best way to do this so it is still configurable from the admin control panel is by changing the 3600 constant to 60 seconds (1 minute)
PHP Code:
//While $feed->feed_freq_hours reads hours,
//it would actually be referring to minutes in this case
} elseif ( ( (time() - ($feed->feed_freq_hours * 60)) > strtotime($feed->feed_last_check) ) && ($_GET['override'] == '') ) {
Once you've made that change you would just setup the time as you normally would for each feed in the control panel, just use minutes instead of hours for the time between imports.
I haven't tested this, but I'm pretty sure it should work.
