set session timeout in webconfig

<configuration>

<system.web>

<sessionState mode="Off|InProc|StateServer|SQLServer"
<!--
Mode : Specifies where to store the session state.
Off : Indicates that session state is not enabled.
InProc : Indicates that session state is stored locally.
StateServer : Indicates that session state is stored on a remote server.
SQLServer : Indicates that session state is stored on the SQL Server.
-->
cookieless="true|false"
<!--
cookieless : Specifies whether sessions without cookies should be used to identify client sessions.
true : Indicates that sessions without cookies should be used.
false : Indicates that sessions without cookies should not be used. The default is false.
-->
timeout="number of minutes"
<!--
timeout : Specifies the number of minutes a session can be idle before it is abandoned. The default is 20.
-->

stateConnectionString="tcpip=server:port"

<!--
stateConnectionString :Specifies the server name and port where session state is stored remotely.
For example, "tcpip=127.0.0.1:42424". This attribute is required when mode is StateServer.
-->
sqlConnectionString="sql connection string"

<!--
sqlConnectionString : Specifies the connection string for a SQL Server.

For example, "data source=localhost;Integrated Security=SSPI;Initial Catalog=northwind".
This attribute is required when mode is SQLServer.
-->
stateNetworkTimeout="number of seconds"/>

<!--
stateNetworkTimeout : When using StateServer mode to store session state,specifies the number of seconds the TCP/IP network connection between the Web server and the state server can be idle before the session is abandoned. The default is 10.
-->
</system.web>

</configuration>