Good question. The links normally remain even after the business membership has been dropped. However, periodically we do scan through the directory and remove dead links, parked pages, and sites that don't fit in the category's anymore due to re-development.
Also note, There's not an alert or management interface for administration so all updates have to be done via the database manually. Because of this it's important that after you add a link to the directory that you notify me via pm and give me the url you added so I can get your listing live as soon as possible for you.
Hope that helps,
Eric Lyon