Session Timout limit not increasing from 20 minutes

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ASP.NET

I am trying to increases session timeout limit in my asp.net application (Global.asax) I have set it to 90 minutes using

Sub Session_OnStart()
    Session.Timeout = 90
End Sub

in my global.asax BUT actually it is not increasing from 20 minutes. I have also made changes to Web.config

system.web>
    sessionState timeout="90"  />

Please advise.