I think once every 30 minutes is fair, we shouldn't have to decrease it. The shoutbox is a great way to get users to look at your shop right there and then, and advertising once every 30 shouldn't disrupt the casual flow of chatter, I think the staff just need to keep better track of time for each user's advertisement. lol. Which I'm going to do from now on.
Though yes, the 30 minute rule does need to be posted in the rules on the shoutbox. I'm surprised it isn't there. ._.