Alerts can be sent and repeated in 15 minute increments.
If you're using Alerts to regulate response times, you can trigger notifications in 15, 30 or 45 minute increments in Version 7.0 or newer.
Severity 1 Issues
You can set Alerts such that when a severity 1 issue is reported, it can trigger an alert every 15 minutes until someone takes action on the issue.
To configure alerts this way, check out the based-on alerts blog post for details.
Information about upgrading to Version 7.0 or newer can be found on the downloads page.