Measuring or calculating session duration is important to see how usual working patterns affect the network. This might include how devices join and then leave the network in short order (causing IP address depletion), or how much each resource is accessed through the course of the day (for example, cloud-based virtual machines left running). 

This information can be used to guide rules on session timeouts. An example of this might be to see that devices at a train station have very short session times, as travelers traverse the platform, or check timetables, and so on. Consideration should be given to a short DHCP lease time compared to the lease time when on board the train's Wi-Fi.

Session duration is not always an immediately available metric. There are a number of software packages that can calculate this, normally through a combination of SNMP outputs.