Look at this answer: Schedule Saved Search Retention which tells you to set
dispatch.ttl = <integer>[p] in the config file
However, this will only affect subsequent runs of the saved search. If you want to change the retention of a search that has already been run, you can "save" the search results. This means that the results will never expire, so you will have to manually delete the results when you don't want them any longer.
In savedsearches.conf, I do not see dispatch.ttl. I do see dispatch.earliesttime and dispatch.latesttime, but these seem to refer to the scope of the search, not the retention of search results. Any other ideas?