>Form authentication timeout is 30 minutes by default. It's in the web.config. Session timeout is set separately (also in the web.config) - I think it's like 20 minutes by default.
>
>Take a look at this post by Scott Guthrie (make sure you read the comments). It's pretty helpful.
>
>http://weblogs.asp.net/scottgu/archive/2005/11/08/430011.aspx

One of the comments in the blog proposes this solution
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The answer to all of your problems is very easy!

Instead of changing the cookie or the session timeout, force a refresh. The easiest way to do this is ... put the following line in the form_load.

Response.AddHeader("Refresh", Convert.ToString((Session.Timeout * 60) - 20));

The best place to put this code is in the Master Page Form Load.
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I'm still going through the comments...