Twitter integration the way Klout works



I’m designing a service similar to Klout, this is, the user’s twitter account is scanned continuously in order to update some sort of metric which is the added value of my service. For this to work my app needs to be approved by the user, but I’m a little bit confused on what to do next. How I ( efficiently ) manage to keep my service in synch with user’s new tweets after he/she has approved the app from the first time?

Thanks in advance.