Here you go open your web.config:

Go/find this section:

<sessionState

            mode="InProc"

            stateConnectionString="tcpip=127.0.0.1:42424"

            sqlConnectionString="data source=127.0.0.1;user id=sa;password="

            cookieless="false"

            timeout="20"

    />

Change timeout="20" to what ever you want it to be.

If this doesn't work  maybe you have another Web.config file that your application is taking its settings from.

First try this and if it doesn't work please let us know of the folder structure