The information in this post may assist you, Basically you can setup how long something should be in a specified bucket, You can say that something can stay in cold for 18 months and then it will automatically be deleted (if no frozen script is specified), However the data will be as old as the Hot/warm time as well, before starting it's journey into cold.
http://answers.splunk.com/answers/114896/splunk-index-retention-based-on-retention-period-only-not-size?page=1&focusedAnswerId=114897#114897
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