EC2 Autoscale policy to terminate instances if not needed but before 1 hour

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http://serverfault.com – I have setup EC2 Autoscale policies along CPU monitors and it's working well. I have occasional bursts that triggers autoscale and more servers are added. If the servers are experiencing a burst that endures 30 minutes and two more instances are added to help with the current jobs I'd like to keep them around for 59 minutes (since I paid for 1 hour) instead of them being terminated when scaling down is triggered. So basically I want to move the checks around the one hour mark from start and see if they are still needed, otherwise shut down. Should I play with cooldown or grace-period? Here (HowTos)