The Idle Time-out monitor for each IIS 8.5 website must be enabled.
The idle time-out attribute controls the amount of time a worker process will remain idle before it shuts down. A worker process is idle if it is not processing requests and no new requests are received.
The purpose of this attribute is to conserve system resources; the default value for idle time-out is 20 minutes.
By default, the World Wide Web (WWW) service establishes an overlapped recycle, in which the worker process to be shut down is kept running until after a new worker process is started.