open up ur  web.confing and set 

<sessionState timeout = "20" mode = "InProc" />  inside the <system.web>
and replace 20 whatever you need  . 
or u can use Session.Timeout = "20"; in global.asax file:under the session_start method 
and you have done  .. :)) 

Anurag,

Check your connection timeout in following areas - IIS

Configures session state settings for the current application.

https://msdn.microsoft.com/en-us/library/h6bb9cz9%28vs.71%29.aspx