The default value is 900 secs (15min) before a session is marked as idle.
This can be changed using below reg. A value of -1 can be set to indicate that the session will never be considered idle.
Registry Hive HKEY_LOCAL_MACHINE
Registry Path Software\Policies\VMware, Inc.\VMware VDM\Agent\Configuration\XMLAPI
Value Name userIdleTimeout
Value Type REG_SZ
Default Value 900
For details check https://kb.vmware.com/s/article/56636
You can see which sessions are idle or for how long with PowerCLI - this sample returns that data: