if you set it to update every 5 min's you might get banned from dyndns.
once a day is enough, you'll find etc/cron.weekly/refresh-ddns
refreshes once a week, maybe you could create a template and put in and does the same as refresh-ddns and put in /etc/cron.daily/ someone might give you a little more incite, I'm fairly new to this my self.
cheers