I could not find this property under $SPLUNK_HOME$/system/default/inputs.conf
* The amount of time, in seconds, that the file monitor must wait for
modifications before closing a file after reaching an End-of-File
* Tells the input not to close files that have been updated in the
past 'timebeforeclose' seconds.
* Default: 3.
I ran a search on all .conf files and it's not defined there as well. It should be hard coded somewhere in the core configuration as this parameter is a core functionality for the monitoring stanza in inputs.conf.