If you're using a Jenkins external monitoring job, you may want to fail the job if it's been too long since the last run. Put this script in a cron tab to check the time since the last successful build. If it's been longer than 2 minutes it will submit a failed job to Jenkins.

jenkins_external_monitor_timeout.sh

#!/bin/bash

# get the last sucessful build
XML=`curl http://localhost:8080/job/External-Monitoring/lastSuccessfulBuild/api/xml`

# extract the first 10 digits of the timestamp
TIMESTAMP=`echo $XML| sed -E 's/.*timestamp>(.*)\<\/timestamp.*/\1/' | cut -c1-10`

NOW=`date +"%s"`

DIFF=$((NOW-TIMESTAMP))

if [[ $DIFF -gt 120 ]] 
then
	HEX=`echo "Haven't seen a sucessful job in over 2 minutes" | xxd -c 256 -p`
	curl -X POST -d "<run><log encoding='hexBinary'>$HEX</log><result>1</result><duration>1</duration></run>" http://localhost:8080/job/External-Monitoring/postBuildResult
	echo "It's been too long since last success: $DIFF"
else 
	echo "ok $DIFF"
fi