The short answer to this question is: as soon as possible.
At the time of writing this (in January 2010), each feed is polled for new items roughly every 30 minutes. All feeds are currently treated as equal. So the same standards apply for every feed added to RSS Graffiti.
We are currently working on a new version of our processing engine that is designed to give us more flexibility in how we schedule processing of feeds. This engine is also designed to be as fast as possible (much faster than the one we are currently using). We hope that this will bring down the feed processing cycle to 10 minutes or less. It will also allow us (and our users) to schedule each feed differently based on how often it is being updated.
This article will have to be updated after the new processing engine is released. The version of RSS Graffiti at the time of writing this is 1.8.0 Beta.
The updated (faster) processing engine will be released with version 1.9.0 Beta. To make sure this article applies to the current version of RSS Graffiti please check the current version number in the application to right corner of the screen here: http://apps.facebook.com/rssgraffiti/
You may also want to see this relevant discussion on our forum: http://getsatisfaction.com/rssgraffiti/topics/what_is_the_frequency_that_rss_graffiti_checks_my_feed_s