I would advise against doing notifications more quickly than this.  In fact, I would have the interval be even longer in many cases.  The notifications are driven by RSS feeds being processed for the most recent events on the system.  The feeds in question will run queries against the backend database.  If you do these too frequently, they just add load to the backend database.  The question to ask is this, "What BUSINESS need is this serving?".  For many customers, they just want the feeling that they are being notified in "real time".  Having notification like this doesn't trigger any other work, it's primarily a communication to encourage collaboration.