controlling timeout

Hi All,
How can the automatic timeout (by which a user gets logged out due to inactivity) be controlled?
I tried this:
<authentication mode="Forms">                    
      <forms timeout="50000000" slidingExpiration="true"/>
      </authentication>
But this does not work. While the code suggests that the user is never timed out (or only after a very very long time), this is not what is happening, I still get a timeout after around 30 min.
Thanks.
Damian

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