How to set Session Timeout in web.config

under system.web you can write session state.

<system.web>
       <sessionState     timeout="60"  />
</system.web>

The session time-out refers to the number of minutes before the user session is aborted. If a large file is being uploaded it is desirable to maintain the session state. The session time-out should always be longer than the amount of time you expect uploads to take. Note that this value is only relevant if you have session state enabled.
In the above web.config file above the sessionState is set to one hour.