This feature is available in 2.2.8 and later versions of Confluence

When macros such as the RSS macro, the Calendar macro or the Repository Client Plugin make HTTP requests to servers which are down, a long timeout value is used. You can set this timeout value through a system parameter to avoid this.

If you are using Tomcat, you can add this to the catalina.bat/sh file:

JAVA_OPTS=-Dhttp.timeout=timeInSecondsHere