We can configure the Application Pool Recycling in two ways.

1. Reactive application pool process recycling.

2. Proactive application pool process recycling.

Reactive Process Recycling:
It happens when process is not functioning properly or unable to serve requests. 
Symptoms: Process deadlocks, access violations, memory leaks, Insufficiant memory and so on, in order to trigger a process recycle.
You can help control the conditions that cause a process restart by using the requestQueueLimit, memoryLimit, and shutdownTimeout configuration attributes.
For more information about these attributes, see the <processModel> Element configuration section of Machine.Config file.

Proactive Process Recycling
Proactive process recycling restarts the worker process periodically even if the process is healthy.
This can be a useful way to prevent denials of service due to conditions the process model is unable to detect.
A process can be restarted after a specific number of requests or after a time-out period has elapsed. You can enable proactive process recycling using the timeout, idleTimeout, and requestLimit configuration attributes. For more information, see the <processModel> Element configuration section of Machine.Config file.

Below are the default settings of Process model found at machine.config file.

<processModel

enable="true"

timeout="Infinite"

idleTimeout="Infinite"

shutdownTimeout="0:00:05"

requestLimit="Infinite"

requestQueueLimit="5000"

restartQueueLimit="10"

memoryLimit="60"

webGarden="false"

cpuMask="0xffffffff"

userName="machine"

password="AutoGenerate"

logLevel="Errors"

clientConnectedCheck="0:00:05"

comAuthenticationLevel="Connect"

comImpersonationLevel="Impersonate"

responseRestartDeadlockInterval="00:09:00"

responseDeadlockInterval="00:03:00"

maxWorkerThreads="20"

maxIoThreads="20"

/>

Satalaj