[Xymon] How to shorten delay before purple alerts?

How can I change xymon to alert after a server has failed to report for 15 minutes? We're finding that 30 minutes is too long a delay in the production environment.

I see where you set purple delay for network tests and I see where you can return it for individual tests, and I see where you can disable purple. Is this a code change?

thanks Betsy