If you are using MVC, you put this in the web.config file in the Root directory of the web application, not the web.config in the Views directory. It also needs to be IN the system.web node, not under like George2 stated in his question: "I wrote under system.web section in the web.config"
The timeout parameter value represents minutes.
There are other attributes that can be set in the sessionState element. You can find information here: docs.microsoft.com sessionState
<configuration>
<system.web>
<sessionState timeout="20"></sessionState>
</system.web>
</configuration>
You can then catch the begining of a new session in the Global.asax file by adding the following method:
void Session_Start(object sender, EventArgs e)
{
if (Session.IsNewSession)
{
//do things that need to happen
//when a new session starts.
}
}