You can define your own device inactivity rule by setting the Last seen within prior time values for the First/Last Seen network filter by selecting one of the following values:
For instance, if the Device Inactivity is set to 10 minutes, the Last seen within prior time values for the First/Last Seen network filter are set as follows:
When viewing devices in the Network tab, the row of any device that is considered inactive will have lighter text than active devices.