I'm serving ArcGIS Desktop concurrent licenses from ArcGIS Administrator 10.3.1. The maximum borrow limit is set to the default 30 days. It seems to me that I once extended that time for one of our installations. A notice in Administrator says to "Contact your License Administrator to modify this value." Administrator Help unhelpfully says "Contact your organization's license administrator to change this value." I tried contacting our license administrator, but that didn't help, because he/she is I. I searched Support, but only found explanations that the maximum time is 65536 days, and warnings that it is best to keep burrowing times short because if a machine crashes or is stolen, the license will be unavailable until the expiration time. I was only thinking about extending the borrow time to 60 days.
It seems like the answer might be in the Options file ACTIVATION_EXPIRY_DAYS parameter http://desktop.arcgis.com/en/arcmap/10.3/get-started/license-manager-guide/activation-expiry-days.htm but for the final paragraph of this usage description: "The ArcGIS Administrator is designed to accept the maximum borrow days specified in the License Server Administrator (default 30 days). The user must adjust the ArcGIS Administrator borrow request for a given feature to that specified in the Options file. "