Why Sesssion expires on local development in Visual Studio in ASP.Net?

Why my Session expires ? even if i had not provided timeout in web.config ? i am using Forms authentication , now after sometime on a page it expires my session , then i tried to give session timeout also but it also won't work .... here's a quick glance of my web.config file :

 <authentication mode="Forms">
      <forms loginUrl="Login.aspx" timeout="30000" />
      <!--timeout="2880"-->
    </authentication>