Simple history retention time
You can edit the simple history retention time in the configuration file : properties.xml located into /opt/datafari/mcf/mcf_home then edit the property  org.apache.manifoldcf.crawler.historycleanupinterval
By default we keep 30 jours of simple history before cleanup.
nano /opt/datafari/mcf/mcf_home/properties.xml
Edit the property :
<property name="org.apache.manifoldcf.crawler.historycleanupinterval" value="2592000000"/>
The unit is millisecond.
Then you have to restart MCF to apply the change.