Mainly this would probably depend on how many people are online and just sit on a certain page or post to the shoutbox. The shoutbox does a check for new content every 30 seconds, and it's a very small check in that it just checks to see if there is anything new in the SHOUT_BOX table.
Where this could get out of hand would be if you have a very large number of people using it at once. But even then it's not a very large resource hog as the queries are much smaller than if those users were actually browsing/making posts.
